IdioMatic

A steal
If something is a steal, it costs much less than it is really worth.
A still tongue keeps a wise head
Wise people don't talk much.
A watched pot never boils
Some things work out in their own time, so being impatient and constantly checking will just make things seem longer.
A1
If something is A1, it is the very best or finest.
Abide by a decision
If you abide by a decision, you accept it and comply with it, even though you might disagree with it.
Abject lesson
(India) An abject lesson serves as a warning to others. (In some varieties of English 'object lesson' is used.)
About as useful as a chocolate teapot
Someone or something that is of no practical use is about as useful as a chocolate teapot.
About face
If someone changes their mind completely, this is an about face. It can be used when companies, governments, etc, change their position on an issue.
Above board
If things are done above board, they are carried out in a legal and proper manner.
Above par
Better than average or normal
Absence makes the heart grow fonder
This idiom means that when people are apart, their love grows stronger.
Accident waiting to happen
If something is an accident waiting to happen, there's definitely going to be an accident or it's bound to go wrong. ('Disaster waiting to happen' is also used.)
Ace in the hole
An ace in the hole is something other people are not aware of that can be used to your advantage when the time is right.