Tall man's regularly ignored regrettably (4)
I believe the answer is:
alas
'regrettably' is the definition.
(I've seen this before)
'tall man's regularly ignored' is the wordplay.
'regularly ignored' means one should take alternating letters.
'tall'+'mans'='tallmans'
The alternate letters of 'tallmans' are 'ALAS'.
(Other definitions for alas that I've seen before include "By bad luck" , "-- and alack!" , "Proper, respectable" , "Sad expression" , "woebegone expression" .)