Complaint of a soldier, say, about love (4)
I believe the answer is:
moan
'complaint' is the definition.
(I've seen this before)
'a soldier say about love' is the wordplay.
'a soldier say' becomes 'man' (soldier is a kind of man).
'about' means one lot of letters goes inside another.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'man' placed around 'o' is 'MOAN'.
'of' acts as a link.
(Other definitions for moan that I've seen before include "Indicate pain" , "Whinge" , "Beef" , "Lament; grumble" , "Bewail" .)