Downer of French worker packing iron (10)
I believe the answer is:
depressant
'downer' is the definition.
('depressant' can be a synonym of 'downer')
'of french worker packing iron' is the wordplay.
'of french' becomes 'de'.
'worker' becomes 'ant' (worker is a type of ant).
'packing' means one lot of letters goes inside another.
'iron' becomes 'press' (I've seen this before).
'de'+'ant'='deant'
'deant' placed around 'press' is 'DEPRESSANT'.
(Other definitions for depressant that I've seen before include "reducing activity of vital functions" , "Drug reducing nervous activity" , "Downer" , "Sedative drug" , "Lowering spirits" .)