In Greece, perhaps, describing Thessaly's capital as lively (9)
I believe the answer is:
energetic
'lively' is the definition.
('energetic' can be similar in meaning to 'lively')
'in greece perhaps describing thessaly's capital' is the wordplay.
'perhaps' indicates anagramming the letters.
'describing' is an insertion indicator (describe can mean to form the outline of).
'capital' says to take the initial letters.
The first letter of 'thessaly' is 't'.
'in'+'greece'='ingreece'
'ingreece' anagrammed gives 'energeic'.
'energeic' enclosing 't' is 'ENERGETIC'.
'as' is the link.
(Other definitions for energetic that I've seen before include "Brisk" , "Full of go" , "'Animated, full of life (9)'" , "Showing great activity" , "Vigorous and dynamic" .)