Time of year seeing one of five bison, perhaps suppressed by Spain (7)
I believe the answer is:
equinox
'time of year' is the definition.
(I've seen this before)
'one of five bison perhaps suppressed by spain' is the wordplay.
'one of five' becomes 'quin' (short for quintuplet, one of five children).
'bison perhaps' becomes 'ox' (ox can mean a bison).
'suppressed by' is a charade indicator (letters next to each other).
'spain' becomes 'e' (abbreviation for Espana).
'quin'+'ox'='quinox'
'quinox' put after 'e' is 'EQUINOX'.
'seeing' is the link.
(Other definitions for equinox that I've seen before include "Time of day/night equality" , "particular day, twice a year" , "Time in March or Sept" , "Vernal - -" , "Biannual event" .)