Splendid fish pursued by one worker (9)
I believe the answer is:
brilliant
'splendid' is the definition.
(I know that splendid can be written as brilliant)
'fish pursued by one worker' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'pursued by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'worker' becomes 'ant' (worker is a type of ant).
'brill'+'i'+'ant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Fantastic!" , "Excellent; a diamond" , "Of outstanding ability" , "Small type" , "Bright and sparkling" .)