Understood thanks, note it! (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'thanks note it' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'note' becomes 'c' (musical note).
'ta'+'c'+'it'='TACIT'
(Other definitions for tacit that I've seen before include "Not said though understood" , "Understood without being directly expressed" , "Implicit, unspoken" , "Silently understood" , "Understood, implicit" .)