Number exercise, we hear, in part of plant (7)
I believe the answer is:
tendril
'part of plant' is the definition.
(I've seen this before)
'number exercise we hear' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'exercise' becomes 'drill' (military exercise).
'we hear' shows a homophone (sound like).
'drill' sounds like 'dril'.
'ten'+'dril'='TENDRIL'
'in' acts as a link.
(Other definitions for tendril that I've seen before include "Plant's climbing aid" , "A creeper, or a curl of hair" , "Clinging part of a plant" , "Climbing plant's supporter" , "Climbing plant's grasper" .)