Component with unknown function (5)
I believe the answer is:
party
'function' is the definition.
(I've seen this before)
'component with unknown' is the wordplay.
'component' becomes 'part' (I've seen this before).
'with' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'part'+'y'='PARTY'
(Other definitions for party that I've seen before include "See 22" , "MP's faction" , "Political group - celebration" , "Bash" , "Social gathering; person" .)