Slow delivery, in all, is failing to secure a maiden over (5,4)
I believe the answer is:
snail mail
'slow delivery' is the definition.
(I've seen this before)
'in all is failing to secure a maiden over' is the wordplay.
'in' is an insertion indicator.
'failing' indicates an anagram.
'to secure' indicates putting letters inside (inserted letters are secured inside).
'a maiden' becomes 'm' (cricket abbreviation).
'over' becomes 'nai' (I can't justify this - if you can you should give a lot more credence to this answer).
'all'+'is'='allis'
'allis' is an anagram of 'slail'.
'slail' placed around 'm' is 'slmail'.
'slmail' going around 'nai' is 'SNAIL MAIL'.
Can you help me to learn more?
(Other definitions for snail mail that I've seen before include "Conventional (slow) post (informal)" , "old-fashioned correspondence?" , "traditional post" , "The conventional postal service" , "Postal system (facetious)" .)