Sea creature grandad accommodates for all to see (7)
I believe the answer is:
grampus
'sea creature' is the definition.
(grampus can mean a killer whale)
'grandad accommodates for all to see' is the wordplay.
'grandad' becomes 'gramps' (synonyms).
'accommodates' is an insertion indicator.
'for all to see' becomes 'u' (short for Universal film certificate).
'gramps' going around 'u' is 'GRAMPUS'.
(Other definitions for grampus that I've seen before include "Good synchronised swimmer" , "Dolphin - killer whale" , "danger in the main" , "huge creature" , "One often spouting" .)