Minds children in small sections crossing gorge briefly (4-4)

'minds children' is the definition.
(to baby-sit is to look after or mind children)

'small sections crossing gorge briefly' is the wordplay.
'small sections' becomes 'bits' (a bit is a small piece).
'crossing' means one lot of letters goes inside another.
'gorge' becomes 'abysm' (an abysm is a vast gorge or gulf).
'briefly' means to remove the last letter.
'abysm' with its final letter taken away is 'abys'.
'bits' placed around 'abys' is 'BABY-SITS'.

'in' acts as a link.