Lucid US soldier enters one's nearest pub (7)
I believe the answer is:
logical
'lucid' is the definition.
(I know that lucid can be written as logical)
'us soldier enters one's nearest pub' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'enters' means one lot of letters goes inside another.
'one's nearest pub' becomes 'local' (I've seen this before).
'gi' put inside 'local' is 'LOGICAL'.
(Other definitions for logical that I've seen before include "Deductive" , "Based on reason" , "Sound" , "Objective" , "entirely reasonable" .)