'Blimey, that's not right!' after drain almost gives way (8)
I believe the answer is:
succumbs
'gives way' is the definition.
'blimey that's not right after drain almost' is the wordplay.
'blimey' becomes 'crumbs' (both are expressions of surprise).
'that's not' is a deletion indicator.
'right' becomes 'r' (common abbreviation).
'after' says to put letters next to each other.
'drain almost' becomes 'suc' (I am not sure about this - if you are sure you should believe this answer much more).
'crumbs' with 'r' taken out is 'cumbs'.
'cumbs' after 'suc' is 'SUCCUMBS'.
(Other definitions for succumbs that I've seen before include "Gives in (to temptation)" , "Yields to pressure, capitulates" , "Gives in, to pressure say" , "Gives way to something overpowering" , "Gives in, yields to" .)