Beermat used periodically for a number of years (3)
I believe the answer is:
era
'number of years' is the definition.
'beermat used periodically for a' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for era that I've seen before include "Long period of time" , "Specific time" , "Age of importance" , "Important time" , "Eon" .)