IoW feature last visited by people making maps of course (8,2,3)
I believe the answer is:
needless to say
'of course' is the definition.
(I've seen this before)
'iow feature last visited by people making maps' is the wordplay.
'iow feature' becomes 'needles' (natural feature of the Isle of Wight).
'last' becomes 'stay' (I've seen this before).
'visited by' means one lot of letters goes inside another.
'people making maps' becomes 'os' (Ordnance Survey, UK mapping agency).
'stay' going around 'os' is 'stosay'.
'needles'+'stosay'='NEEDLESS TO SAY'
(Another definition for needless to say that I've seen is " Of course".)