I had to bypass hole in urban area in rain (3,2,4)

'rain' is the definition.
('it's tipping it down' means 'it's raining')

'i had to bypass hole in urban area in' is the wordplay.
'i had' becomes 'I'd' (common contraction).
'to bypass' means one lot of letters goes inside another (some letters go round or bypass others).
'hole in' becomes 'pit' (pit is a kind of hole. I am not sure about the 'in' bit.).
'urban area' becomes 'town' (town is a kind of urban area).
'in' is an insertion indicator.
'id' placed around 'pit' is 'ipitd'.
'ipitd' going inside 'town' is 'TIP IT DOWN'.