A girl is too short, sad to say (4)
I believe the answer is:
alas
'sad to say' is the definition.
('alas' is said before reporting something sad)
'a girl is too short' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'is too short' means to remove the last letter.
'lass' with its final letter taken away is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "What a shame!" , "woebegone expression" , "Unhappily" , "Sad to say!" , "Cry of woe" .)