Shy type, mostly ever so tense, after opening (9)
I believe the answer is:
introvert
'shy type' is the definition.
(I've seen this before)
'mostly ever so tense after opening' is the wordplay.
'mostly' means to remove the first letter.
'ever so' becomes 'ever' ('ever' can be a synonym of 'ever so').
'tense' becomes 't' (abbreviation used in many dictionaries).
'after' is a charade indicator (letters next to each other).
'opening' becomes 'intro' (I've seen this before**).
'ever'+'t'='evert'
'evert' with its first letter taken away is 'vert'.
'vert' put after 'intro' is 'INTROVERT'.
(Other definitions for introvert that I've seen before include "Withdrawn type" , "one's reserved" , "Turn inwards" , "Rover tint (anag.)" , "eschewing external contact" .)