Regularly felt it gently upend (4)
I believe the answer is:
flip
'upend' is the definition.
Both the answer and definition are verbs in their base form.
Perhaps there's a link between them I don't understand?
'regularly felt it gently' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'gently' becomes 'p' (musical abbreviation for 'piano', softly).
'felt'+'it'='feltit'
The alternate letters of 'feltit' are 'fli'.
'fli'+'p'='FLIP'
Can you help me to learn more?
(Other definitions for flip that I've seen before include "Not showing respect; turn over" , "Somersault - glib - hot drink" , "Turn (over) - impudent" , "Turn quickly over; sort of chart" , "Toss - flick - turn over" .)