Caught by the rain, knocked outside (7)
I believe the answer is:
trapped
'caught' is the definition.
(trapping is a kind of catching)
'the rain knocked outside' is the wordplay.
'the rain' becomes 'r' (this might be a standard abbreviation I've not previously seen).
'knocked' becomes 'tapped' ('tap' can be a synonym of 'knock').
'outside' means one lot of letters goes inside another.
'r' going inside 'tapped' is 'TRAPPED'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for trapped that I've seen before include "Stuck" , "Ensnared, cornered" , "Snared" , "in a tight corner" , "Caught" .)