Traversing cold Antarctic sea wearing gilet top (8)
I believe the answer is:
crossing
'traversing' is the definition.
('cross' can be a synonym of 'traverse')
'cold antarctic sea wearing gilet top' is the wordplay.
'cold' becomes 'C' (eg on taps).
'antarctic sea' becomes 'ross' (Ross Sea).
'wearing' becomes 'in' (you are 'in' the clothes you're wearing).
'top' says to take the initial letters (in a down clue, the first letter is top).
The initial letter of 'gilet' is 'g'.
'c'+'ross'+'in'+'g'='CROSSING'
(Other definitions for crossing that I've seen before include "trip on ferry?" , "Passage" , "Where to go to the other side of the road" , "Journey over water" , "Junction" .)