Being upset by pals can be a blow (4)
I believe the answer is:
slap
'a blow' is the definition.
(slap is a kind of blow)
'being upset by pals' is the wordplay.
'being upset by' is an anagram indicator.
'pals' anagrammed gives 'SLAP'.
'can be' is the link.
(Other definitions for slap that I've seen before include "Hit with an open hand" , "Mild punishment" , "Hit with the palm" , "Blow with open hand" , "Strike action" .)