Let down, as dipped into irregularly (12)
I believe the answer is:
disappointed
'let down' is the definition.
(to let someone down is to disappoint them)
'as dipped into irregularly' is the wordplay.
'irregularly' is an anagram indicator.
'as'+'dipped'+'into'='asdippedinto'
'asdippedinto' is an anagram of 'DISAPPOINTED'.
(Other definitions for disappointed that I've seen before include "Let down" , "Feeling sad" , "Unhappy" , "Let-down, frustrated" , "Sad and let-down" .)