Fail to mention rise in testimonials (4)
I believe the answer is:
omit
'fail to mention' is the definition.
(I've seen this before)
'rise in testimonials' is the wordplay.
'rise' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'timo' can be found hidden inside 'testimonials'.
'timo' back-to-front is 'OMIT'.
(Other definitions for omit that I've seen before include "Drop" , "Fail to mention" , "Fail to use" , "Leave out or overlook" , "Leave something out" .)