Promote retro technology, in hostile environment (9)
I believe the answer is:
advertise
'promote retro technology' is the definition.
'advertise' can be an answer for 'promote' (thesaurus). I am unsure of the 'retro technology' bit.
'in hostile environment' is the wordplay.
'in' indicates putting letters inside.
'hostile' becomes 'adverse' (I've seen this before).
'environment' becomes 'ti' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'adverse' placed around 'ti' is 'ADVERTISE'.
(Other definitions for advertise that I've seen before include "Promote (a product)" , "Publicize, call attention to" , "Market" , "Make public what is for sale" , "Praise - plug" .)