Demanding old fellow takes one in (7)
I believe the answer is:
exigent
'demanding' is the definition.
('exigent' can be similar in meaning to 'demanding')
'old fellow takes one in' is the wordplay.
'old' becomes 'ex'.
'fellow' becomes 'gent' ('gent' can be a synonym of 'fellow'**).
'takes' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'ex'+'gent'='exgent'
'exgent' placed around 'i' is 'EXIGENT'.
(Other definitions for exigent that I've seen before include "Pressing; demanding" , "Urgent and taxing" , "Exacting, requiring much" , "Urgent and exacting" , "Pressing and urgent" .)