Trip away in the middle of the night (6)
I believe the answer is:
outing
'trip' is the definition.
(I've seen this before)
'away in the middle of the night' is the wordplay.
'away' becomes 'out' (synonyms).
'the middle of' means to look at the middle letters.
The centre of 'night' is 'g'.
'out'+'in'+'g'='OUTING'
(Other definitions for outing that I've seen before include "Day excursion" , "Jaunt" , "A trip in the open air" , "that puts the private on parade?" , "journey" .)