Heartily enjoy mum's sisters' outings (6)
I believe the answer is:
jaunts
'outings' is the definition.
('jaunt' can be a synonym of 'outing')
'heartily enjoy mum's sisters'' is the wordplay.
'heartily' means to look at the middle letters.
'mum's sisters'' becomes 'aunts'.
The central letter of 'enjoy' is 'j'.
'j'+'aunts'='JAUNTS'
(Other definitions for jaunts that I've seen before include "Short pleasure trips" , "'Junkets, pleasure trips (6)'" .)