Pressing on gamely at first with complaint in minutes (7)
I believe the answer is:
milling
'pressing on' is the definition.
(milling can mean pressing on or throning)
'gamely at first with complaint in minutes' is the wordplay.
'at first' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'complaint' becomes 'ill' (synonyms).
'in' means one lot of letters goes inside another.
'minutes' becomes 'min' (short for minutes).
The first letter of 'gamely' is 'g'.
'ill' put within 'min' is 'millin'.
'g' after 'millin' is 'MILLING'.
(Other definitions for milling that I've seen before include "Grinding - (of crowd) moving about in a confused manner" , "Edge of a coin" , "crushing" .)