Indefinite number above number that's lower (6)
I believe the answer is:
nether
'that's lower' is the definition.
(synonyms)
'indefinite number above number' is the wordplay.
'indefinite number' becomes 'n' (mathematical symbol).
'above' says to put letters next to each other.
'number' becomes 'ether' (I've seen this before).
'n'+'ether'='NETHER'
(Other definitions for nether that I've seen before include "Located below" , "Subordinate" , "These regions are hell, the underworld" , "North European country" , "Underground" .)