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' says the letters should be written backwards.
'sunscreen' with its middle 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" .)