Lawrence emptied trawler - one fish! (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'lawrence emptied trawler one' is the wordplay.
'lawrence' becomes 'TE' (initials of TE Lawrence aka Lawrence of Arabia).
'emptied' means to remove the middle letters.
'one' becomes 'a' (a thing is one thing).
'trawler' with its centre taken out is 'tr'.
'te'+'tr'+'a'='TETRA'
(Other definitions for tetra that I've seen before include "Denoting four (prefix)" , "starter for four" , "rarely seen flower" , "Brightly coloured aquarium fish" , "Wrongful act (Law)" .)