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?' says to hollow out the word (remove centre letters) (the inside letters must vacate).
'irrational' becomes 'pi' (an irrational number).
'looking back' shows that the letters should be reversed in order.
'sunscreen' with its centre taken out is 'sn'.
'pi' back-to-front is '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" .)