Flattened boss to catch the leader (8)
I believe the answer is:
nailhead
'flattened boss' is the definition.
The definition suggests a singular noun which matches the answer.
'catch the leader' is the wordplay.
'catch' becomes 'nail' (nail can mean to catch or arrest).
'the leader' becomes 'head' (head is a kind of leader).
'nail'+'head'='NAILHEAD'
'to' is the link.
Can you help me to learn more?
(Other definitions for nailhead that I've seen before include "As hit by the carpenter" , "that may come under the hammer" , "Decorative device; had alien (anagram)" , "Ornament; where the hammer hits" .)