Grumbles if humanity's without love (5)
I believe the answer is:
moans
'grumbles' is the definition.
(I've seen this before)
'humanity's without love' is the wordplay.
'humanity's' becomes 'man's' ('man' can be a synonym of 'humanity').
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'love' becomes 'o' (love means zero in tennis).
'mans' going around 'o' is 'MOANS'.
'if' acts as a link.
(Other definitions for moans that I've seen before include "Complains" , "- - and groans" , "Mournful sounds" , "Grumbles" , "Expressions of unhappiness" .)