Every now and then Manx cat's tail begins to stretch (3)
I believe the answer is:
tax
'stretch' is the definition.
(both can mean to make heavy demands on)
'every now and then manx cat's tail begins' is the wordplay.
'every now and then' indicates alternate letters (letters selected at regular intervals).
'tail' says to take the final letters.
'begins' says to put letters next to each other.
The last letter of 'cat' is 't'.
The alternate letters of 'manx' are 'ax'.
'ax' put after 't' is 'TAX'.
'to' acts as a link.
(Other definitions for tax that I've seen before include "Payment to the Exchequer" , "Are demanding" , "Levy; make demands on" , "Be trying" , "Compulsory contribution towards a country's expenses" .)