Expressionless and very plain, oddly (7)
I believe the answer is:
deadpan
'expressionless' is the definition.
(synonyms)
'very plain oddly' is the wordplay.
'very' becomes 'dead' (dead can mean very or absolutely, eg 'dead wrong').
'oddly' indicates alternate letters (letters in odd-numbered positions).
The alternating letters of 'plain' are 'pan'.
'dead'+'pan'='DEADPAN'
'and' acts as a link.
(Other definitions for deadpan that I've seen before include "Impassive" , "with emotionless face" , "Blank" , "Not giving anything away" , "Dry" .)