Forceful chap I met at sea (8)
I believe the answer is:
emphatic
'forceful' is the definition.
(similar in meaning)
'chap i met at sea' is the wordplay.
'at sea' is an anagram indicator.
'chap'+'i'+'met'='chapimet'
'chapimet' anagrammed gives 'EMPHATIC'.
(Other definitions for emphatic that I've seen before include "being stressed" , "For sure" , "Forceful and definite" , "Categorical" , "In no uncertain terms" .)