Lithium and iron sparkle (4)
I believe the answer is:
life
'sparkle' is the definition.
(sparkle can mean life or liveliness)
'lithium and iron' is the wordplay.
'lithium' becomes 'Li' (chemical symbol).
'and' means one lot of letters go next to another.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'li'+'fe'='LIFE'
(Other definitions for life that I've seen before include "Birth to death" , "Baffin oiler (anag) - 7 film" , "'While there's .... there's hope'" , "Vitality, vigour" , "Maximum stretch?" .)