Endure lampoonery, losing heart in the end (6)
I believe the answer is:
lastly
'in the end' is the definition.
(I've seen this before)
'endure lampoonery losing heart' is the wordplay.
'endure' becomes 'last' ('last' can be a synonym of 'endure'**).
'losing heart' means to remove the middle letters.
'lampoonery' with its middle removed is 'ly'.
'last'+'ly'='LASTLY'
(Other definitions for lastly that I've seen before include "In the end" , "In conclusion" , "Finally, to conclude with" .)