Move ten in unknown number (6)
I believe the answer is:
ninety
'number' is the definition.
(ninety is a kind of number)
'move ten in unknown' is the wordplay.
'move' indicates anagramming the letters (I've seen 'moving' mean this).
'unknown' becomes 'y' (algebra).
'ten'+'in'='tenin'
'tenin' with letters rearranged gives 'ninet'.
'ninet'+'y'='NINETY'
(Other definitions for ninety that I've seen before include "Cardinal" , "high number" , "Minutes played in a football match" , "Three thirties" , "Number of degrees in a right angle" .)