Engineers, fellows this person's put in fighting units (9)
I believe the answer is:
regiments
'fighting units' is the definition.
(regiment is a kind of military unit)
'engineers fellows this person's put in' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'fellows' becomes 'gents' ('gent' can be a synonym of 'fellow'**).
'this person's' becomes 'im' (this person means the crossword setter themselves).
'put in' is an insertion indicator.
'gents' placed around 'im' is 'giments'.
're'+'giments'='REGIMENTS'
(Other definitions for regiments that I've seen before include "fighting units" , "Organises" , "groups of soldiers" .)