Avoid fish and fowl when young (8)
I believe the answer is:
duckling
'fowl when young' is the definition.
(young duck)
'avoid fish' is the wordplay.
'avoid' becomes 'duck' (duck can mean to avoid).
'fish' becomes 'ling' (ling is a kind of fish).
'duck'+'ling'='DUCKLING'
'and' is the link.
(Other definitions for duckling that I've seen before include "Young swimming bird" , "Young mallard, for instance" , "little fowl" , "Young waterfowl" , "Young swimmer" .)