Designate for a particular purpose (5)
(Other definitions for allot that I've seen before include "Give as a share, or a task to be done" , "Apportion (something to someone)" , "Distribute in portions" , "Earmark" , "Assign (to someone)" .)
'designate for a particular purpose' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for earmark that I've seen before include "Reserve specially" , "Set aside, designate" , "'Reserve, set aside (7)'" , "Set aside for future use" , "reserve for future use" .)