Blue unit will regularly pulse (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'blue unit will regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'blue'+'unit'+'will'='blueunitwill'
The alternate letters of 'blueunitwill' are 'LENTIL'.
(Other definitions for lentil that I've seen before include "Leguminous vegetable" , "Edible leguminous plant seed" , "Small seed used for food" , "Kind of bean plant used in soup" , "Small edible seed" .)