Confound Edward being undersized! (7)
I believe the answer is:
stunted
'undersized' is the definition.
(I've seen this before)
'confound edward' is the wordplay.
'confound' becomes 'stun' (I've seen this in another clue).
'edward' becomes 'ted' ().
'stun'+'ted'='STUNTED'
'being' acts as a link.
(Other definitions for stunted that I've seen before include "Retarded in growth" , "Dwarfed, limited" , "Undersized" .)