Splurge when in Paris at end of June, visiting sister (8)
I believe the answer is:
squander
'splurge' is the definition.
(thesaurus)
'when in paris at end of june visiting sister' is the wordplay.
'when in paris' becomes 'quand' ('when' in French).
'at end of' indicates one should take the final letters.
'visiting' is an insertion indicator.
'sister' becomes 'sr' (abbreviation as in names of nuns).
The last letter of 'june' is 'e'.
'quand'+'e'='quande'
'quande' going into 'sr' is 'SQUANDER'.
(Other definitions for squander that I've seen before include "Waste (a resource) recklessly" , "Spend wastefully" , "Dissipate" , "blow" , "Misspend" .)