Bird entering weakest relationship (7)
I believe the answer is:
kestrel
'bird' is the definition.
(kestrel is a kind of bird)
'entering weakest relationship' is the wordplay.
'entering' indicates a hidden word.
'KESTREL' can be found hidden inside 'weakest relationship'.
(Other definitions for kestrel that I've seen before include "Predator" , "It takes mice" , "Bird of prey" , "High flier" , "High-flyer" .)