Fish, medium-sized -- a kilo in creel, wriggling (8)
I believe the answer is:
mackerel
'fish' is the definition.
(mackerel is a kind of fish)
'medium-sized a kilo in creel wriggling' is the wordplay.
'medium-sized' becomes 'm' (clothes size).
'kilo' becomes 'k' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'wriggling' is an anagram indicator.
'creel' anagrammed gives 'cerel'.
'k' placed inside 'cerel' is 'ckerel'.
'm'+'a'+'ckerel'='MACKEREL'
(Other definitions for mackerel that I've seen before include "Spiny-finned food fish" , "Common striped fish, very tasty" , "A food fish; type of sky" , "Oily food fish" , "Swimmer" .)