Pulse needs hour -- put in to blanch at first (5)
I believe the answer is:
throb
'pulse' is the definition.
(I know that pulse can be written as throb)
'hour put in to blanch at first' is the wordplay.
'hour' becomes 'hr' (abbreviation).
'put in' is an insertion indicator.
'at first' indicates taking the first letters.
The initial letter of 'blanch' is 'b'.
'hr' inserted into 'to' is 'thro'.
'thro'+'b'='THROB'
'needs' acts as a link.
(Other definitions for throb that I've seen before include "Beat quickly from excitement etc" , "Pulsate with force" , "Pulsate; regular beat" , "Strong regular beat" , "Beat strongly" .)