Stylish Tessa coming out of her shell, outwardly dull (6)
I believe the answer is:
dressy
'stylish' is the definition.
(I've seen this before)
'tessa coming out of her shell outwardly dull' is the wordplay.
'coming out of her shell' says to take the centre (the outside letters or 'shell' are removed).
'outwardly' indicates putting letters inside (some letters go outside others).
'dull' becomes 'dry' (I've seen this before).
The centre of 'tessa' is 'ess'.
'ess' placed into 'dry' is 'DRESSY'.
(Other definitions for dressy that I've seen before include "Elaborate" , "'Stylish, smart (6)'" , "Stylishly attired" , "Smartly clad, wearing stylish clothes" , "Formal or elegant" .)