Squire worried about lover's heart tremors (7)
I believe the answer is:
quivers
'tremors' is the definition.
('quiver' can be a synonym of 'tremor')
'squire worried about lover's heart' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'about' indicates putting letters inside.
'heart' means to look at the middle letters.
The central letter of 'lover' is 'v'.
'squire' is an anagram of 'quiers'.
'quiers' going around 'v' is 'QUIVERS'.
(Other definitions for quivers that I've seen before include "Trembles" , "Cases for holding arrows" .)