They let out zebras regularly to chase on fields (9)
I believe the answer is:
releasers
'they let out' is the definition.
'zebras regularly to chase on fields' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'to' says to put letters next to each other (I've seen this in other clues).
'chase on' becomes 're' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fields' becomes 'leas' (I've seen this before).
The alternate letters of 'zebras' are 'ers'.
're'+'leas'='releas'
'ers' put after 'releas' is 'RELEASERS'.
(Other definitions for releasers that I've seen before include "instigators of liberation?" , "They've lost their grip" .)