In retirement, Mrs Thatcher having to admit 'time is a four-letter word' (9)
I believe the answer is:
tetragram
'is a four-letter word' is the definition.
(I've seen this before)
'in retirement mrs thatcher having to admit time' is the wordplay.
I cannot really see how this works, but
'mrs' could be 'ag' and 'ag' is present in the answer.
'time' could be 'am' and 'am' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tetragram that I've seen before include "Four-letter word" , "familiar to geometricians !" , "Say damn (but not blast)" .)