Formerly of some concern (4)
I believe the answer is:
once
'formerly' is the definition.
('once' can be a synonym of 'formerly')
'some concern' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ONCE' is hidden in the letters of 'concern'.
'of' is the link.
(Other definitions for once that I've seen before include "One-time - one time" , "Previously" , "Formerly; as soon as" , "Cone (anag.)" , "On a single (or past) occasion" .)