Engineer free to move in jacket (6)
I believe the answer is:
reefer
'jacket' is the definition.
(I've seen this in another clue)
'engineer free to move in' is the wordplay.
'engineer' becomes 're' (Royal Engineer).
'to move' indicates an anagram.
'in' means one lot of letters goes inside another.
'free' anagrammed gives 'efer'.
're' put inside 'efer' is 'REEFER'.
(Other definitions for reefer that I've seen before include "Cigarette containing cannabis - jacket" , "habit?" , "close-fitting garment" , "refrigerated transport" , "Hand-rolled cigarette" .)