Wearing nothing, good to go in for jab (5)
I believe the answer is:
nudge
'for jab' is the definition.
(thesaurus)
'wearing nothing good to go in' is the wordplay.
'wearing nothing' becomes 'nude' (in the nude).
'good' becomes 'g' (abbreviation).
'to go in' means one lot of letters goes inside another.
'nude' going around 'g' is 'NUDGE'.
(Other definitions for nudge that I've seen before include "Gentle encouragement" , "Gentle push" , "Prod gently with elbow" , "Encouraging prompt" , "Festive entertainment" .)