One in a boat — gentleman given cheer standing up (6)
I believe the answer is:
harris
'one in a boat' is the definition.
(character from the novel Three Men in a Boat)
'gentleman given cheer standing up' is the wordplay.
'gentleman' becomes 'sir' (a man may be addressed as sir).
'given' means one lot of letters go next to another.
'cheer' becomes 'rah' (rah is a triumphant cry).
'standing up' says the letters should be written backwards (in a down clue up is reverse order).
'sir'+'rah'='sirrah'
'sirrah' backwards is 'HARRIS'.
(Other definitions for harris that I've seen before include "Hebridean area associated with tweed" , "Cloth from Scottish island" , "Joel Chandler --, creator of Brer Rabbit" , "location off Scottish mainland" , "boating man" .)