Party leader in charge of endlessly pleasant outing (6)
I believe the answer is:
picnic
'outing' is the definition.
('picnic' can be a synonym of 'outing')
'party leader in charge of endlessly pleasant' is the wordplay.
'party leader' becomes 'p' (I've seen this in other clues).
'in charge' becomes 'ic'.
'of endlessly' means to remove the last letter (I've seen 'endlessly' mean this).
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'nice' with its final letter taken off is 'nic'.
'p'+'ic'+'nic'='PICNIC'
(Other definitions for picnic that I've seen before include "Meal eaten outside" , "Money for jam" , "Alfresco meal" , "Piece of cake!" , "Meal outdoors" .)