Implicitly understood (5)
I believe the answer is:
tacit
'implicitly understood' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tacit that I've seen before include "Implicit, unspoken" , "Understood without being put into words" , "Not said though understood" , "Implied -- inferred" , "Aircraft engine casing" .)