One newt in the sequence is the leader (9)
I believe the answer is:
chieftain
'the leader' is the definition.
(chieftain is a kind of leader)
'one newt in the sequence' is the wordplay.
'one' becomes 'i' (Roman numeral).
'newt' becomes 'eft' (eft is a kind of newt).
'in the' means one lot of letters goes inside another.
'sequence' becomes 'chain' (I've seen this before).
'i'+'eft'='ieft'
'ieft' going inside 'chain' is 'CHIEFTAIN'.
'is' is the link.
(Other definitions for chieftain that I've seen before include "Tribal chief" , "Head of tribe" , "Clan leader" , "Tribal leader" , "Tribe's leader" .)