It tails a wildcat, that's understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'it tails a wildcat' is the wordplay.
'it' becomes 'tag' (playground catching game).
'tails' means to remove the last letter (I've seen 'tailed' mean this (to tail something can mean to remove its end)).
'a wildcat' becomes 'cit' (I can't justify this - if you can you should believe this answer much more).
'tag' with its last letter removed is 'ta'.
'ta'+'cit'='TACIT'
'that's' is the link.
Can you help me to learn more?
(Other definitions for tacit that I've seen before include "Implicitly understood" , "Understood, implicit" , "Implied or understood, not put into words" , "Understood, not spoken or played" , "Understood without being directly expressed" .)