Double back, lad, out of turn! (4)
I believe the answer is:
dual
'double' is the definition.
(synonyms)
'back lad out of turn' is the wordplay.
'back' says the letters should be written backwards.
'out of' means one lot of letters goes inside another.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'lad' placed around 'u' is 'laud'.
'laud' reversed gives 'DUAL'.
(Other definitions for dual that I've seen before include "Consisting of pairs" , "Consisting of two parts" , "Having two distinct functions" , "Road with central division" , "Twofold" .)