P.I. Alec's problem birth brings important mail (7,8)
I believe the answer is:
special delivery
'important mail' is the definition.
'special delivery' can be an answer for 'mail' (special delivery is a kind of mail). I'm not sure about the 'important' bit.
'p i alec's problem birth' is the wordplay.
'problem' indicates anagramming the letters.
'birth' becomes 'delivery' (delivery is a kind of birth).
'p'+'i'+'alecs'='pialecs'
'pialecs' anagrammed gives 'special'.
'special'+'delivery'='SPECIAL DELIVERY'
'brings' is the link.
Can you help me to learn more?
(Other definitions for special delivery that I've seen before include "What a courier may have brought" , "that a spinner hopes to produce?" .)