Fireman back shortly, fine to go in (6)
I believe the answer is:
stoker
'fireman' is the definition.
(someone who tends to a furnace)
'back shortly fine to go in' is the wordplay.
'back' says the letters should be written backwards.
'shortly' becomes 'rets' (I am not sure about this - if you are sure you should believe this answer much more).
'fine' becomes 'ok' (synonyms).
'to go in' is an insertion indicator.
'rets' reversed gives 'ster'.
'ster' going around 'ok' is 'STOKER'.
(Other definitions for stoker that I've seen before include "Vampires were subject to" , "piling on the heat?" , "Someone who fuels a furnace" , "who added fuel to the flames?" , "He's fuelled < the legend of Dracula" .)