Specialist catch with line at sea (9)
I believe the answer is:
technical
'specialist' is the definition.
(I've seen this before)
'catch with line at sea' is the wordplay.
'with' is a charade indicator (letters next to each other).
'at sea' indicates anagramming the letters.
'catch'+'line'='catchline'
'catchline' with letters rearranged gives 'TECHNICAL'.
(Other definitions for technical that I've seen before include "sort of college" , "Requiring specialist knowledge" , "Scientific" , "specialising in applied science" , "Concerned with applied sciences" .)