Slight earthquake in heart of Castries, revolutionary capital (6)
I believe the answer is:
tremor
'slight earthquake' is the definition.
(I've seen this before)
'heart of castries revolutionary capital' is the wordplay.
'heart of' indicates the central letters.
'revolutionary' says the letters should be written backwards.
'capital' becomes 'rome' (Rome is an example).
The middle of 'castries' is 'tr'.
'rome' back-to-front is 'emor'.
'tr'+'emor'='TREMOR'
'in' is the link.
(Other definitions for tremor that I've seen before include "Vibration from earthquake" , "feeling of excitement" , "Minor earthquake" , "make one shake" , "Nervousness" .)