Fine-tune variable way of working on holiday (9)
I believe the answer is:
honeymoon
'holiday' is the definition.
(honeymoon is a kind of holiday)
'fine-tune variable way of working on' is the wordplay.
'fine-tune' becomes 'hone'.
'variable' becomes 'y' (I've seen this in other clues).
'way of working' becomes 'mo' (I've seen this in other clues).
'hone'+'y'+'mo'+'on'='HONEYMOON'
(Other definitions for honeymoon that I've seen before include "Sequel to marriage" , "Holiday of a newly married couple" , "Introductory period" , "Post-reception break" , "Holiday just for two?" .)