Tom comes up with it - that's understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'tom comes up with it' is the wordplay.
'tom' becomes 'cat' (a tom is a male cat).
'comes up' shows that the letters should be reversed in order (in a down clue, letters go up).
'with' says to put letters next to each other.
'cat' reversed gives 'tac'.
'tac'+'it'='TACIT'
'that's' acts as a link.
(Other definitions for tacit that I've seen before include "Implicit, unspoken" , "Implied or understood, not put into words" , "Aircraft engine casing" , "Understood without being put into words" , "Unvoiced" .)