Bird with muscles kept turning round, looking primarily for bouncer (6,4)
I believe the answer is:
tennis ball
'bouncer' is the definition.
(tennis balls bounce)
'bird with muscles kept turning round looking primarily' is the wordplay.
'bird with' becomes 'linnet' (linnet is a kind of bird. I am not sure about the 'with' bit.).
'muscles' becomes 'abs' (short for abdominals).
'kept' means one lot of letters goes inside another.
'turning round' says the letters should be written backwards.
'primarily' suggests taking the first letters.
The initial letter of 'looking' is 'l'.
'linnet' placed around 'abs' is 'labsinnet'.
'labsinnet' written backwards gives 'tennisbal'.
'tennisbal'+'l'='TENNIS-BALL'
'for' is the link.
(Other definitions for tennis ball that I've seen before include "Participant in rally" , "Bouncer" , "Item hit with rackets" , "Something hit by racquets" , "Item for Wimbledon game" .)