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 really see how this works, but
'a' is within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
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" .)