Space to deal with unknown plea (8)
I believe the answer is:
entreaty
'plea' is the definition.
(I've seen this before)
'space to deal with unknown' is the wordplay.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'to deal with' becomes 'treat' ('treat' can be a synonym of 'deal with').
'unknown' becomes 'y' (algebra).
'en'+'treat'+'y'='ENTREATY'
(Other definitions for entreaty that I've seen before include "Teeny rat (anag.)" , "application for assistance" , "Earnest or urgent request" , "Supplication" , "Earnest plea" .)