Against upsetting no-good giant? (4)
I believe the answer is:
anti
'against' is the definition.
(prefix meaning against)
'upsetting no-good giant?' is the wordplay.
'upsetting' indicates anagramming the letters.
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'giant' with 'g' removed is 'iant'.
'iant' with letters rearranged gives 'ANTI'.
(Other definitions for anti that I've seen before include "Adversary" , "One who is against (something)" , "one opposing?" , "Against, informally" , "not favoured" .)