More certain about female boarder (6)
I believe the answer is:
surfer
'boarder' is the definition.
(I've seen this before)
'more certain about female' is the wordplay.
'more certain' becomes 'surer' ('sure' can be similar in meaning to 'certain').
'about' means one lot of letters goes inside another.
'female' becomes 'f' (abbreviation).
'surer' going around 'f' is 'SURFER'.
(Other definitions for surfer that I've seen before include "See 7" , "One riding on the crest of a wave?" , "Boarder" , "Wave rider" , "Board rider" .)