Ultimately, muscle behind lift (5)
I believe the answer is:
elate
'lift' is the definition.
(thesaurus)
'ultimately muscle behind' is the wordplay.
'ultimately' says to take the final letters.
'behind' becomes 'late'.
The last letter of 'muscle' is 'e'.
'e'+'late'='ELATE'
(Other definitions for elate that I've seen before include "Fill with joy" , "Encourage" , "Make happy or joyful" , "transport?" , "Lift (the spirits)" .)