Many a health tracker (3)
I believe the answer is:
app
'many a health tracker' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for app that I've seen before include "Software item" , "bit of software" , "Software for mobile devices, in short" , "Software program for a mobile device" , "Program for a smart phone, in short" .)