Like a plain fish secured by lock (8)
I believe the answer is:
treeless
'like a plain' is the definition.
(I've seen this before)
'fish secured by lock' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish).
'secured by' is an insertion indicator (inserted letters are held or secured).
'lock' becomes 'tress' (both can mean a piece of hair).
'eel' going into 'tress' is 'TREELESS'.
(Other definitions for treeless that I've seen before include "As the Nullarbor Plain (if its name is to be believed)" , "Like a plain" , "out of planes?" , "like down, say" , "With no very large plants" .)