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' shows that the letters should be reversed in order.
'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' going around 'u' is 'laud'.
'laud' written backwards 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" .)