Complaint from relative, old at heart (5)
I believe the answer is:
groan
'complaint' is the definition.
(to groan is to complain about something)
'relative old at heart' is the wordplay.
'relative' becomes 'gran' (I've seen this before).
'old at' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'heart' means one lot of letters goes inside another.
'gran' placed around 'o' is 'GROAN'.
'from' is the link.
Can you help me to learn more?
(Other definitions for groan that I've seen before include "Titus -- (Mervyn Peake)" , "Sound of one in pain" , "painful sigh" , "Make pained sound" , "Complain" .)