Row hard in extremely grimy boat (6)
I believe the answer is:
dinghy
'boat' is the definition.
(type of boat)
'row hard in extremely grimy' is the wordplay.
'row' becomes 'din' (both can mean a loud disturbance).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in' is an insertion indicator.
'extremely' says to hollow out the word (remove centre letters) (letters on the extremes of the word are used).
'grimy' with its middle taken out is 'gy'.
'h' placed into 'gy' is 'ghy'.
'din'+'ghy'='DINGHY'
(Other definitions for dinghy that I've seen before include "Small open boat, maybe a rubber inflatable" , "Small boat propelled with oars" , "Small open boat, may be inflatable" , "Small rowing or sailing boat" , "Small boat, may be a rubber inflatable" .)