Go away – it's cloudless on holiday (5,3)
I believe the answer is:
clear off
'go away' is the definition.
(I've seen this before)
'cloudless on holiday' is the wordplay.
'cloudless' becomes 'clear' (I've seen this in another clue).
'on holiday' becomes 'off' (eg on holiday from work).
'clear'+'off'='CLEAR OFF'
'it's' acts as a link.
(Other definitions for clear off that I've seen before include "Go away - transparently" , "Begone (colloq.)" , "Skedaddle" , "Leave" , "Run along!" .)