Department concerned with charitable donations, mostly (5)
I believe the answer is:
realm
'department' is the definition.
(I've seen this before)
'concerned with charitable donations mostly' is the wordplay.
'concerned with' becomes 're' (re can mean regarding, concerning).
'charitable donations' becomes 'alms' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'alms' with its final letter taken away is 'alm'.
're'+'alm'='REALM'
(Other definitions for realm that I've seen before include "Area of influence" , "Kingdom's" , "Kingdom, domain" , "the UK, perhaps" , "Empire - state" .)