Miners get retrained as fighting units (9)
I believe the answer is:
regiments
'fighting units' is the definition.
(regiment is a kind of military unit)
'miners get retrained' is the wordplay.
'retrained' indicates an anagram (train can mean to shape or direct a plant).
'miners'+'get'='minersget'
'minersget' is an anagram of 'REGIMENTS'.
'as' is the link.
(Other definitions for regiments that I've seen before include "Organises" , "groups of soldiers" , "fighting units" .)