Ring on small target for bird (6)
I believe the answer is:
osprey
'bird' is the definition.
(osprey is a kind of bird)
'ring on small target' is the wordplay.
'ring' becomes 'o'.
'on' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'target' becomes 'prey' ('prey' can be a synonym of 'target'**).
'o'+'s'+'prey'='OSPREY'
'for' is the link.
(Other definitions for osprey that I've seen before include "Marine bird of prey" , "One flying" , "Large bird preying on fish in inland waters" , "Piscivorous creature" , "Fish eagle" .)