Regularly fervent with worry (4)
I believe the answer is:
fret
'worry' is the definition.
(fretting is a kind of worrying)
'regularly fervent' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'fervent' are 'FRET'.
'with' acts as a link.
(Other definitions for fret that I've seen before include "Worry or agitate" , "Notes separator" , "Be constantly anxious" , "Be anxious; ornament" , "Harry - on the guitar" .)