Serious little creature swallowing soldier at sea (6)
I believe the answer is:
formal
'serious' is the definition.
(thesaurus)
'little creature swallowing soldier at sea' is the wordplay.
'little creature' becomes 'foal' (baby horse).
'swallowing' is an insertion indicator.
'soldier at sea' becomes 'rm' (Royal Marine).
'foal' placed around 'rm' is 'FORMAL'.
(Other definitions for formal that I've seen before include "Done in accordance with etiquette" , "Ceremonious or prescribed" , "Regular" , "Ceremonial; ordered" , "black tie" .)