Tom comes back 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 back with it' is the wordplay.
'tom' becomes 'cat' (a tom is a male cat).
'comes back' is a reversal indicator.
'with' says to put letters next to each other.
'cat' in reverse letter order is 'tac'.
'tac'+'it'='TACIT'
'that's' acts as a link.
(Other definitions for tacit that I've seen before include "Silent but implied" , "Wordless" , "Implied without being put into words" , "Implied -- inferred" , "Silent - understood" .)