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 in reverse.
'out of' indicates putting letters inside (some letters on the outside of others).
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'lad' enclosing 'u' is 'laud'.
'laud' backwards is '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" .)