A part extremely lively, suitably (5)
I believe the answer is:
aptly
'suitably' is the definition.
(I've seen this in another clue)
'a part extremely lively' is the wordplay.
'part' becomes 'pt' (abbreviation).
'extremely' suggests removing the centre (only the extremes of the word are used).
'lively' with its centre taken out is 'ly'.
'a'+'pt'+'ly'='APTLY'
(Other definitions for aptly that I've seen before include "Suitably" , "Fittingly" , "Appropriately" , "In a competent manner" , "In suitable form" .)