Spirit of additional ace Labour leader's in... (6)
I believe the answer is:
morale
'spirit' is the definition.
(thesaurus)
'additional ace labour leader's in' is the wordplay.
'additional' becomes 'more' ('more' can be a synonym of 'additional').
'leader's' indicates taking the first letters (I've seen 'leaders' mean this).
'in' is an insertion indicator.
The first letters of 'ace labour' is 'al'.
'more' enclosing 'al' is 'MORALE'.
'of' acts as a link.
(Other definitions for morale that I've seen before include "Degree of optimism (of troops, perhaps)" , "Esprit, degree of confidence" , "Esprit de corps" , "State of mental confidence" , "Self-confidence" .)