Top lieutenant accepts soldiers' complaint (7)
I believe the answer is:
ailment
'complaint' is the definition.
(I know that complaint can be written as ailment)
'top lieutenant accepts soldiers'' is the wordplay.
'top' becomes 'ai' (resembles A1, term for excellent).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'accepts' means one lot of letters goes inside another.
'soldiers'' becomes 'men' (men can mean a group of soldiers).
'lt' going around 'men' is 'lment'.
'ai'+'lment'='AILMENT'
(Other definitions for ailment that I've seen before include "Physical disorder" , "Tummy upset, for instance" , "Slight but persistent illness" , "Malady" , "Complaint, affliction" .)