Fix one's heart trouble (4)
I believe the answer is:
nail
'fix' is the definition.
(thesaurus)
'one's heart trouble' is the wordplay.
'one's' becomes 'uns' (I've seen this in other clues).
'heart' says to take the centre.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
The centre of 'uns' is 'n'.
'n'+'ail'='NAIL'
(Other definitions for nail that I've seen before include "Measure of cloth" , "one under the hammer" , "Growth on digit" , "Fix with a metal spike" , "Apprehend" .)