Rounded shape, place to go over when climbing (4)
I believe the answer is:
oval
'rounded shape' is the definition.
(an oval is a round shape)
'place to go over when climbing' is the wordplay.
'place to go' becomes 'lav' (short for lavatory, somewhere to 'go').
'over' becomes 'o' (cricket abbreviation).
'when climbing' says the letters should be written in reverse.
'lav'+'o'='lavo'
'lavo' written backwards gives 'OVAL'.
(Other definitions for oval that I've seen before include "Shape of 25" , "London cricket ground" , "Shaped like an egg" , "34 [CRICKETER] 's domain" , "sports ground" .)