Athlone is a source of alcohol (7)
I believe the answer is:
ethanol
'alcohol' is the definition.
(ethanol is a kind of alcohol)
'athlone is a source' is the wordplay.
'is a source' indicates an anagram.
'athlone' anagrammed gives 'ETHANOL'.
'of' is the link.
(Other definitions for ethanol that I've seen before include "porter uses it" , "Kind of alcohol found in Athlone" , "Alcohol in wine, beer" , "spirit" , "Ethyl alcohol" .)