Opening pub is very fine, but to no purpose (2,4)
I believe the answer is:
in vain
'to no purpose' is the definition.
(I've seen this before)
'opening pub is very fine' is the wordplay.
'opening' is an insertion indicator (some letters open a gap in others).
'pub' becomes 'inn' (an inn is a type of pub).
'is very' becomes 'v' (abbreviation).
'fine' becomes 'ai' (resembles A1, informal term for top quality).
'v'+'ai'='vai'
'inn' going around 'vai' is 'IN VAIN'.
'but' acts as a link.
(Other definitions for in vain that I've seen before include "Uselessly, without result" , "With no profitable outcome, a waste of time" , "Uselessly, a waste of time" , "Without effect" , "Uselessly, to no profit" .)