Cut sunscreen after vacation? Irrational, looking back (4)
I believe the answer is:
snip
'cut' is the definition.
(snip means to cut)
'sunscreen after vacation? irrational looking back' is the wordplay.
'after vacation?' means to remove the middle letters (I've seen 'after vacation' mean this (the inside letters must vacate)).
'irrational' becomes 'pi' (an irrational number).
'looking back' says the letters should be written backwards.
'sunscreen' with its centre taken out is 'sn'.
'pi' written backwards gives 'ip'.
'sn'+'ip'='SNIP'
(Other definitions for snip that I've seen before include "Quick cut - something easily done" , "Bargain - crop" , "Cut with shears" , "Bargain (coll)" , "Cut - certainty" .)