Shy person beginning to get, for the most part, really tense (9)
I believe the answer is:
introvert
'shy person' is the definition.
(I've seen this before)
'beginning to get for the most part really tense' is the wordplay.
'beginning' becomes 'intro' (I've seen this before**).
'to get' says to put letters next to each other.
'for the most part' means to remove the last letter.
'really' becomes 'very' (I've seen this before).
'tense' becomes 't' (abbreviation used in many dictionaries).
'very' with its last letter taken off is 'ver'.
'intro'+'ver'+'t'='INTROVERT'
(Other definitions for introvert that I've seen before include "One who's withdrawn" , "one's reserved" , "Withdrawn type" , "Self-absorbed person" , "eschewing external contact" .)