Engineers free up joint (6)
I believe the answer is:
reefer
'joint' is the definition.
(informal term for a marijuana joint)
'engineers free up' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'up' indicates anagramming the letters.
'free' with letters rearranged gives 'efer'.
're'+'efer'='REEFER'
(Other definitions for reefer that I've seen before include "Cigarette containing cannabis - jacket" , "Type of jacket - smoke" , "Hand-rolled cigarette" , "A wool jacket; a spliff" , "cold vehicle" .)