Highly regard including unknown trade union in amalgamation process (9)
I believe the answer is:
admixture
'amalgamation process' is the definition.
(admixture is a process of mixing or amalgamating)
'highly regard including unknown trade union' is the wordplay.
'highly regard' becomes 'admire'.
'including' indicates putting letters inside.
'unknown' becomes 'x' (algebra).
'trade union' becomes 'tu' (abbreviation for trade union).
'x'+'tu'='xtu'
'admire' going around 'xtu' is 'ADMIXTURE'.
'in' is the link.
(Other definitions for admixture that I've seen before include "amalgamation" , "Ingredient" , "little extra" .)