Heedless bull is to heed at intervals (6)
I believe the answer is:
blithe
'heedless' is the definition.
(I've seen this before)
'bull is to heed at intervals' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'bull'+'is'+'to'+'heed'='bullistoheed'
The alternating letters of 'bullistoheed' are 'BLITHE'.
(Other definitions for blithe that I've seen before include "Joyful and unconcerned" , "Careless" , "Happy, without worries or cares" , "Heedless" , "'' ... Spirit'', Noel Coward play" .)