Usually single, concealing a grave problem (2,7)
I believe the answer is:
on average
'usually' is the definition.
(I've seen this before)
'single concealing a grave problem' is the wordplay.
'single' becomes 'one' (a single thing - only one).
'concealing' means one lot of letters goes inside another.
'problem' indicates an anagram.
'a'+'grave'='agrave'
'agrave' anagrammed gives 'averag'.
'one' going around 'averag' is 'ON AVERAGE'.
(Other definitions for on average that I've seen before include "typically speaking" , "Generally" , "Usually" .)