Fish worker hauls in one diamond (9)
I believe the answer is:
brilliant
'diamond' is the definition.
(thesaurus)
'fish worker hauls in one' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'worker' becomes 'ant' (worker is a type of ant).
'hauls in' means one lot of letters goes inside another (some letters are caught or hauled in).
'one' becomes 'i' (Roman numeral).
'brill'+'ant'='brillant'
'brillant' going around 'i' is 'BRILLIANT'.
(Other definitions for brilliant that I've seen before include "Of outstanding ability" , "Of surpassing excellence" , "Brainy" , "Glittering" , "Fantastic!" .)