Maiden secured by one fairly good ball -- never mind the others! (2,3,5,4)
I believe the answer is:
im all right jack
'never mind the others' is the definition.
(I've seen this before)
'maiden secured by one fairly good ball' is the wordplay.
'maiden' becomes 'm' (cricket abbreviation).
'secured by' indicates putting letters inside (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'fairly good' becomes 'allright'.
'ball' becomes 'jack' (type of ball in bowls).
'm' inserted within 'iallright' is 'imallright'.
'imallright'+'jack'='IM ALL RIGHT JACK'
(Other definitions for im all right jack that I've seen before include "Selfish expression" , "Selfish comment" , "Smug" , "Reassurance from Jill after accident" , "words to partner after fetching water from tumbler?" .)