In twice in raging sea? That's senseless (7)
I believe the answer is:
asinine
'senseless' is the definition.
(I've seen this before)
'in twice in raging sea?' is the wordplay.
'in twice' means two replacements for 'in'.
'in' is an insertion indicator.
'raging' indicates anagramming the letters.
'in'+'in'='inin'
'sea' with letters rearranged gives 'ase'.
'inin' put into 'ase' is 'ASININE'.
'that's' is the link.
(Other definitions for asinine that I've seen before include "Senseless" , "Stupid (as a donkey?)" , "Stubborn" , "Foolish like donkey" , "Is Annie? (Anag)" .)