Cousin's mother (4)
I believe the answer is:
aunt
'cousin's mother' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aunt that I've seen before include "Tuna gone bad for the relative" , "Close relative" , "Family relative" , "Charley's ___ , farce staged in London in 1892, revived and adapted ever since" , "Family member" .)