Excited exclamation Dorothy regularly let out (3)
I believe the answer is:
ooh
'excited exclamation' is the definition.
(exclamation of surprise or interest)
'dorothy regularly let out' is the wordplay.
'regularly let out' means one should take alternating letters (letters removed at regular intervals).
The alternating letters of 'dorothy' are 'OOH'.
(Other definitions for ooh that I've seen before include "Exclamation of delight" , "My goodness!" , "sound excited" , "Expression of surprise" , "Exclamation of surprise or pain" .)