Somehow turn a gag about a name for giant (10)
I believe the answer is:
gargantuan
'for giant' is the definition.
(synonyms)
'somehow turn a gag about a name' is the wordplay.
'somehow' indicates anagramming the letters.
'about' is a charade indicator (letters next to each other).
'name' becomes 'n' (common abbreviation e.g. on forms).
'turn'+'a'+'gag'='turnagag'
'turnagag' put after 'a' is 'aturnagag'.
'aturnagag' anagrammed gives 'gargantua'.
'gargantua'+'n'='GARGANTUAN'
(Other definitions for gargantuan that I've seen before include "Elephantine" , "Enormous" , "Stupendously big" , "Prodigious" , "Huge" .)