Climb, emerging topless (6)
I believe the answer is:
ascent
'climb' is the definition.
('ascent' can be a synonym of 'climb')
'emerging topless' is the wordplay.
'emerging' becomes 'nascent' ('nascent' can be a synonym of 'emerging').
'topless' means to remove the first letter.
'nascent' with its initial letter taken away is 'ASCENT'.
(Other definitions for ascent that I've seen before include "Upward journey" , "Upward climb" , "A climb or way up" , "Incline" , "Act of moving upwards" .)