Drat, the rain's coming in, comrade! (7)
I believe the answer is:
brother
'comrade' is the definition.
(I know that comrade can be written as brother)
'drat the rain's coming in' is the wordplay.
'drat' becomes 'bother' (I've seen this before).
'the rain's' becomes 'r' (this could be a standard abbreviation I've not previously seen).
'coming in' is an insertion indicator.
'bother' placed around 'r' is 'BROTHER'.
Can you help me to learn more?
(Other definitions for brother that I've seen before include "Eg, monk, friar" , "Fellow-member" , "Male sibling" , "Union member - male relative" , "Close relative" .)