Put a strain on (3)
I believe the answer is:
tax
'put a strain on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tax that I've seen before include "Be trying" , "Payment to the Exchequer" , "E.g. VAT" , "Strain - duty" , "Compulsory contribution towards a country's expenses" .)