Attempt to take in bloke with old idea (6)
I believe the answer is:
theory
'idea' is the definition.
(I know that theory is a type of concept)
'attempt to take in bloke with old' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'take in' means one lot of letters goes inside another.
'bloke' becomes 'he' ('he' refers to a man).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'he'+'o'='heo'
'try' going around 'heo' is 'THEORY'.
(Other definitions for theory that I've seen before include "Set of principles on which practice is based" , "Explanation or system of anything" , "Ideal situation" , "Proposed explanation of something" , "Thought" .)