Messenger entertains tense attendant (8)
I believe the answer is:
courtier
'attendant' is the definition.
(courtier is a kind of attendant)
'messenger entertains tense' is the wordplay.
'messenger' becomes 'courier' (synonyms).
'entertains' is an insertion indicator.
'tense' becomes 't' (abbreviation used in many dictionaries).
'courier' placed around 't' is 'COURTIER'.
(Other definitions for courtier that I've seen before include "royal companion" , "Monarch's companion" , "Royal adviser" , "Setter" , "Messenger" .)