Love interrupting elderly relative's lament (5)
I believe the answer is:
groan
'lament' is the definition.
(thesaurus)
'love interrupting elderly relative's' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'interrupting' means one lot of letters goes inside another.
'elderly relative' becomes 'gran' (short for grandmother).
'o' going inside 'gran' is 'GROAN'.
(Other definitions for groan that I've seen before include "Sound of pain or disapproval" , "Moan, grumble" , "Organ (anag) - lament" , "Complain" , "Sound pained; Gormenghast earldom" .)