A regiment, forced to develop (9)
I believe the answer is:
germinate
'develop' is the definition.
(I know that develop can be written as germinate)
'a regiment forced' is the wordplay.
'forced' is an anagram indicator.
'a'+'regiment'='aregiment'
'aregiment' with letters rearranged gives 'GERMINATE'.
'to' is the link.
(Other definitions for germinate that I've seen before include "Mere giant (anag) - start to grow" , "Start to grow - a regiment (anag)" , "(Of seeds) sprout" , "Spring up" , "Put forth shoots, develop and grow" .)