Recruits being regularly on the field of combat once (7)
I believe the answer is:
enlists
'recruits' is the definition.
(enlisting is a kind of recruiting)
'being regularly on the field of combat once' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'on' is a charade indicator (letters next to each other).
'the field of combat once' becomes 'lists' (lists could mean the field in jousting contests).
The alternate letters of 'being' are 'en'.
'en'+'lists'='ENLISTS'
(Other definitions for enlists that I've seen before include "Joins up" , "volunteers" , "gets involved" , "Signs up" , "Listens (anag.)" .)