Vague old boy gets awful curse (7)
I believe the answer is:
obscure
'vague' is the definition.
(I know that vague can be written as obscure)
'old boy gets awful curse' is the wordplay.
'old boy' becomes 'ob' (abbreviation for a private school 'old boy').
'gets' means one lot of letters goes inside another.
'awful' is an anagram indicator.
'curse' anagrammed gives 'scure'.
'ob' going into 'scure' is 'OBSCURE'.
(Other definitions for obscure that I've seen before include "Hard to make out" , "Insignificant" , "Unclear - recondite" , "Not clearly expressed" , "Gloomy" .)