Lively one kept in iron pen (6)
I believe the answer is:
feisty
'lively' is the definition.
'one kept in iron pen' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' is an insertion indicator.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'pen' becomes 'sty' (sty is a kind of pen).
'fe'+'sty'='festy'
'i' placed within 'festy' is 'FEISTY'.
(Other definitions for feisty that I've seen before include "Confident, high-spirited (US)" , "Spirited -- argumentative" , "Spirited and lively" , "American word for aggressive or combative" , "full of fighting spirit" .)