Rate at which ovule takes pressure (5)
I believe the answer is:
speed
'rate at' is the definition.
'speed' can be an answer for 'rate' (rate at which something is moving). I am unsure of the 'at' bit.
'ovule takes pressure' is the wordplay.
'ovule' becomes 'seed' (seed is a kind of ovule).
'takes' indicates putting letters inside.
'pressure' becomes 'p' (abbreviation).
'seed' enclosing 'p' is 'SPEED'.
'which' acts as a link.
Can you help me to learn more?
(Other definitions for speed that I've seen before include "Pace or tempo" , "Film sensitivity to light" , "Expedition" , "Rate of progress" , "Swiftness" .)