Acceptable to enter number, or symbol (5)
I believe the answer is:
token
'symbol' is the definition.
(token is a kind of symbol)
'acceptable to enter number' is the wordplay.
'acceptable' becomes 'OK' (OK can mean acceptable or fine).
'to enter' means one lot of letters goes inside another.
'number' becomes 'ten' (ten is a kind of number).
'ok' placed inside 'ten' is 'TOKEN'.
'or' is the link.
(Other definitions for token that I've seen before include "Representative item" , "Symbol; metal disc" , "Indication, symbol or voucher" , "Sign, symbol or an evidence" , "Money substitute" .)