Tolerate a tender sweetheart (5)
I believe the answer is:
abide
'tolerate' is the definition.
('abide' can be a synonym of 'tolerate')
'a tender sweetheart' is the wordplay.
'tender' becomes 'bid' ('bid' can be a synonym of 'tender').
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'a'+'bid'+'e'='ABIDE'
(Other definitions for abide that I've seen before include "Wait for" , "Endure, tolerate" , "Hack" , "Put up with" , "'. . . . . with me, fast falls the eventide'" .)