Really big fish and terribly gaunt man head off (10)
I believe the answer is:
gargantuan
'really big' is the definition.
(I know this)
'fish and terribly gaunt man head off' is the wordplay.
'fish' becomes 'gar' (type of gar).
'and' says to put letters next to each other.
'terribly' is an anagram indicator.
'head off' means to remove the first letter (the first letter or 'head' is removed).
'man' with its initial letter taken away is 'an'.
'gaunt' with letters rearranged gives 'gantu'.
'gar'+'gantu'+'an'='GARGANTUAN'
(Other definitions for gargantuan that I've seen before include "Huge" , "Prodigious" , "Elephantine" , "Massive" , "Enormous" .)