The last character to join in Conservative element (4)
I believe the answer is:
zinc
'element' is the definition.
(zinc is a kind of element)
'the last character to join in conservative' is the wordplay.
'the last character' becomes 'z'.
'to join' says to put letters next to each other.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'z'+'in'+'c'='ZINC'
(Other definitions for zinc that I've seen before include "Galvanizing metal" , "Type of metal" , "trace element" , "White metallic element, symbol Zn" , "Constituent of brass" .)