O for one damned exercise that involves little time (5,4)
I believe the answer is:
blood type
'o for one' is the definition.
(o is a kind of blood type)
'damned exercise that involves little time' is the wordplay.
'damned' becomes 'bloody' (I can't justify this - if you can you should give a lot more credence to this answer).
'exercise' becomes 'pe' (Physical Education).
'that involves' is an insertion indicator.
'little time' becomes 't' (abbreviation for time).
'bloody'+'pe'='bloodype'
'bloodype' going around 't' is 'BLOOD TYPE'.
Can you help me to learn more?
(Other definitions for blood type that I've seen before include "A or B, for example?" , "Font Used By Vampires?" , "such as Auden or Betjeman for starters" , "This might be A" .)