Complain when love concealed by relative (5)
I believe the answer is:
groan
'complain' is the definition.
(thesaurus)
'love concealed by relative' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'concealed by' is an insertion indicator.
'relative' becomes 'gran' (I've seen this in other clues**).
'o' inserted into 'gran' is 'GROAN'.
'when' is the link.
(Other definitions for groan that I've seen before include "painful sigh" , "Complaint" , "Sound of one in pain" , "Cry of pain or despair" , "Organ (anag) - lament" .)