Strength of donkey before effort ends (5)
I believe the answer is:
asset
'strength' is the definition.
(thesaurus)
'donkey before effort ends' is the wordplay.
'donkey' becomes 'ass' (I've seen this before).
'ends' suggests the final letters.
The last letters of 'before effort' are 'et'.
'ass'+'et'='ASSET'
'of' acts as a link.
(Other definitions for asset that I've seen before include "It's an advantage to declare" , "Strong point" , "Something well worth having" , "valuable sort" , "blessing" .)