Break cooking vessels when climbing (4)
I believe the answer is:
snap
'break' is the definition.
(snapping is a kind of breaking)
'cooking vessels when climbing' is the wordplay.
'cooking vessels' becomes 'pans'.
'when climbing' is a reversal indicator (in a down clue, letters go up).
'pans' written backwards gives 'SNAP'.
(Other definitions for snap that I've seen before include "Break; bite suddenly" , "unexpected? > Same here!" , "A walk in the park < to shoot" , "Crack; bite" , "piece of cake" .)