Obtuse angle below centre of boat (6)
I believe the answer is:
oafish
'obtuse' is the definition.
(thesaurus)
'angle below centre of boat' is the wordplay.
'angle' becomes 'fish' (synonyms).
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'centre of' indicates the central letters.
The middle letters of 'boat' are 'oa'.
'fish' after 'oa' is 'OAFISH'.
(Other definitions for oafish that I've seen before include "Doltish" , "Uncouth" , "Loutish" , "Bone-headed" , "Like a boorish person" .)