One enthusiast receiving positive information (5)
I believe the answer is:
input
'information' is the definition.
(input is a kind of information)
'one enthusiast receiving positive' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'receiving' means one lot of letters goes inside another.
'positive' becomes 'p'.
'nut' going around 'p' is 'nput'.
'i'+'nput'='INPUT'
(Other definitions for input that I've seen before include "Data entry" , "Data contributed" , "Contribution; enter (data)" , "Person's contribution (to activity); electrical signal" , "supply" .)