Elated former partner initially usurping lieutenant and worker (8)
I believe the answer is:
exultant
'elated' is the definition.
(similar in meaning)
'former partner initially usurping lieutenant and worker' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'initially' suggests taking the first letters.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'and' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant).
The first letter of 'usurping' is 'u'.
'ex'+'u'+'lt'+'ant'='EXULTANT'
(Other definitions for exultant that I've seen before include "Triumphant" , "really happy" , "In jubilant mood, rejoicing" , "Triumphantly elated" , "Rejoicing greatly, delighted" .)