Number of deliveries bowled with it, rather poorly (5,4)
I believe the answer is:
birth rate
'number of deliveries' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'bowled with it rather poorly' is the wordplay.
'bowled' becomes 'b' (cricket abbreviation).
'with' is a charade indicator (letters next to each other).
'poorly' indicates anagramming the letters.
'it'+'rather'='itrather'
'itrather' is an anagram of 'irthrate'.
'b'+'irthrate'='BIRTH-RATE'
Can you help me to learn more?
(Other definitions for birth rate that I've seen before include "How many children arriving" , "Factor affecting population size" .)