Tell that many fit ones keep up (6)
I believe the answer is:
notify
'tell' is the definition.
(I've seen this before)
'many fit ones keep up' is the wordplay.
'keep' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'yfiton' is hidden within 'many fit ones'.
'yfiton' back-to-front is 'NOTIFY'.
'that' acts as a link.
(Other definitions for notify that I've seen before include "Declare" , "Formally tell" , "Formally advise" , "Formally inform" , "Officially inform" .)