Difficulty for British leaving en masse initially (7)
I believe the answer is:
problem
'difficulty' is the definition.
(problem is a kind of difficulty)
'for british leaving en masse initially' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'initially' indicates taking the first letters.
The first letters of 'british leaving en masse' is 'blem'.
'pro'+'blem'='PROBLEM'
(Other definitions for problem that I've seen before include "Something that needs solving" , "Snag" , "One requiring solution" , "Something to be solved" , "Matter hard to cope with" .)