Positive response protects a source of revenue for ages (5)
I believe the answer is:
years
'ages' is the definition.
(I've seen this before)
'positive response protects a source of revenue' is the wordplay.
'positive response' becomes 'yes' (I've seen this before).
'protects' means one lot of letters goes inside another.
'source of' says to take the initial letters.
The first letter of 'revenue' is 'r'.
'a'+'r'='ar'
'yes' placed around 'ar' is 'YEARS'.
'for' acts as a link.
(Other definitions for years that I've seen before include "Units of time" , "Periods of time" , "12 month periods" , "long period" , "Age" .)