Threesome with time on new moon (6)
I believe the answer is:
triton
'moon' is the definition.
(I've seen this before)
'threesome with time on new' is the wordplay.
'threesome' becomes 'trio' (synonyms).
'with' is an insertion indicator.
'time' becomes 't' (abbreviation).
'on' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'trio' placed around 't' is 'trito'.
'trito'+'n'='TRITON'
(Other definitions for triton that I've seen before include "conch-blowing deity" , "Satellite of Neptune - minor sea god" , "Minor Greek god" , "Trot in after sea-god, son of Poseidon" , "heavenly conch-blower" .)