Pretending to be about to retire can start to gall (6)
I believe the answer is:
acting
'pretending to be' is the definition.
(acting is pretending to be a character)
'about to retire can start to gall' is the wordplay.
'about to' becomes 'ca' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'retire' shows that the letters should be reversed in order.
'can' becomes 'tin' (synonyms).
'start to' suggests taking the first letters.
The first letter of 'gall' is 'g'.
'ca' in reverse letter order is 'ac'.
'ac'+'tin'+'g'='ACTING'
(Other definitions for acting that I've seen before include "Behaving like a player" , "Behaving in a certain way" , "Behaving like thespian" , "Taking a part - on a temporary basis" , "stagecraft" .)