Mark cause extremists espoused as somewhat lacking (6)
I believe the answer is:
scarce
'lacking' is the definition.
(thesaurus)
'mark cause extremists' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'extremists' says to hollow out the word (remove centre letters) (letters on the extreme ends of the word).
'cause' with its centre taken out is 'ce'.
'scar'+'ce'='SCARCE'
'espoused as somewhat' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for scarce that I've seen before include "Falling short of demand" , "Deficient in quantity" , "Few and far between" , "Not enough to satisfy demand" , "Not often found" .)