Nothing after Mayday? Nothing to write home about (2-2)
I believe the answer is:
so-so
'nothing to write home about' is the definition.
(I've seen this before)
'nothing after mayday?' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'after' says to put letters next to each other.
'mayday?' becomes 'sos' (I've seen this in other clues).
'o' put after 'sos' is 'SO-SO'.
(Other definitions for so-so that I've seen before include "Of average quality" , "Informally, only moderately good or well" , "'Passable, middling (2-2)'" , "just about adequate" , "Tolerable, indifferent" .)