Head of garrison more disposed to accept new infantryman (9)
I believe the answer is:
grenadier
'infantryman' is the definition.
(grenadier is a kind of infantryman)
'head of garrison more disposed to accept new' is the wordplay.
'head of' says to take the initial letters.
'more disposed' becomes 'readier'.
'to accept' is an insertion indicator.
'new' becomes 'n'.
The first letter of 'garrison' is 'g'.
'readier' enclosing 'n' is 'renadier'.
'g'+'renadier'='GRENADIER'
(Other definitions for grenadier that I've seen before include "historical bomber" , "guard" , "A household infantryman" , "Soldier - in red gear (anag)" , "African flier" .)