Capone’s problem, concealing Yankee’s bloomer? (7)
I believe the answer is:
alyssum
'bloomer?' is the definition.
(flowering plant)
'capone's problem concealing yankee's' is the wordplay.
'capone' becomes 'Al' (gangster Al Capone).
'problem' becomes 'sum' (arithmetic problem).
'concealing' means one lot of letters goes inside another.
'yankee's' becomes 'Y's' (phonetic alphabet: alpha, bravo, charlie etc.).
'al'+'sum'='alsum'
'alsum' placed around 'ys' is 'ALYSSUM'.
(Other definitions for alyssum that I've seen before include "Asylums (anag.)" , "Rock garden or bedding plant" , "Herbaceous plant" , "Slum, say (anag) - garden plant" , "Rockery plant" .)