Make Union pay church to accept introduction of economic strike in recession (3,3,4)
I believe the answer is:
tie the knot
'make union' is the definition.
(to tie the knot is to marry ie form a union)
'pay church to accept introduction of economic strike in recession' is the wordplay.
'pay church' becomes 'tithe' (to tithe is to pay money to the Church).
'to accept' means one lot of letters goes inside another.
'introduction of' indicates taking the first letters.
'strike' becomes 'tonk' (informal term meaning to hit).
'in recession' shows that the letters should be reversed in order.
The first letter of 'economic' is 'e'.
'tonk' written backwards gives 'knot'.
'tithe' going around 'e' is 'tiethe'.
'tiethe'+'knot'='TIE THE KNOT'
(Other definitions for tie the knot that I've seen before include "Marry" , "Get hitched" , "Get married (informal)" , "Make a Windsor" .)