Beef? Older relative eats duck (5)
I believe the answer is:
groan
'beef?' is the definition.
(thesaurus)
'older relative eats duck' is the wordplay.
'older relative' becomes 'gran' (I've seen this before**).
'eats' indicates putting letters inside.
'duck' becomes 'o'.
'gran' enclosing 'o' is 'GROAN'.
(Other definitions for groan that I've seen before include "Moan, grumble" , "Titus -- (Mervyn Peake)" , "Sound pained; Gormenghast earldom" , "Complain" , "See 18 Down" .)