Conservative legislation covering leader of union in plant (6)
I believe the answer is:
cactus
'plant' is the definition.
(cactus is a kind of plant)
'conservative legislation covering leader of union' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'legislation' becomes 'acts' (I've seen this before).
'covering' is an insertion indicator.
'leader of' says to take the initial letters.
The initial letter of 'union' is 'u'.
'acts' placed around 'u' is 'actus'.
'c'+'actus'='CACTUS'
'in' is the link.
(Other definitions for cactus that I've seen before include "Plant found in dry places" , "Spiky desert plant" , "Desert flower" , "Thorny desert plant" , "Drought-resistant plant" .)