Enigmatic old boy steady outside clubs (7)
I believe the answer is:
obscure
'enigmatic' is the definition.
(synonyms)
'old boy steady outside clubs' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'steady' becomes 'sure' (similar in meaning).
'outside' means one lot of letters goes inside another.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'sure' enclosing 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Dim, vague or not well known" , "Dark - secret - inconspicuous" , "Little-known" , "Dim, indistinct or not well known" , "Like Jude, not well known" .)