Reduce diet regularly and stop eating rubbish, primarily (8)
I believe the answer is:
decrease
'reduce' is the definition.
(I know that reduce is a more specific form of the action decrease)
'diet regularly and stop eating rubbish primarily' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'and' says to put letters next to each other.
'stop' becomes 'cease' (synonyms).
'eating' means one lot of letters goes inside another.
'primarily' suggests taking the first letters.
The initial letter of 'rubbish' is 'r'.
The alternating letters of 'diet' are 'de'.
'cease' placed around 'r' is 'crease'.
'de'+'crease'='DECREASE'
(Other definitions for decrease that I've seen before include "Deminish, lessen" , "Decline" , "Turn down" , "Reduction" , "Iron out the wrinkles" .)