Recruits register to fill spaces (7)
I believe the answer is:
enlists
'recruits' is the definition.
(enlisting is a kind of recruiting)
'register to fill spaces' is the wordplay.
'register' becomes 'list' (listing is a kind of registering).
'to fill' is an insertion indicator.
'spaces' becomes 'ens' (in printing, spaces as wide as a letter 'n').
'list' going into 'ens' is 'ENLISTS'.
(Other definitions for enlists that I've seen before include "volunteers" , "Signs up" , "Recruits" , "Joins up" , "gets involved" .)