Assemble revolutionary missile when outside (5)
I believe the answer is:
amass
'assemble' is the definition.
(I've seen this before)
'revolutionary missile when outside' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'missile' becomes 's am' (abbreviation for surface-to-air missile).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'outside' is an insertion indicator.
'sam' in reverse letter order is 'mas'.
'mas' inserted into 'as' is 'AMASS'.
(Other definitions for amass that I've seen before include "Get together" , "Accumulate, pile up" , "store" , "Stockpile" , "Accumulate, heap up" .)