Ailment dispatching commanding officer? How awful! (3)
I believe the answer is:
ugh
'how awful' is the definition.
(expression of disgust)
'ailment dispatching commanding officer?' is the wordplay.
'ailment' becomes 'cough' (a cough is a medical complaint).
'dispatching' indicates named letters should be taken away.
'commanding officer?' becomes 'OC' (abbreviation for officer commanding).
'cough' with 'oc' taken out is 'UGH'.
(Other definitions for ugh that I've seen before include "Yuck (colloq.)" , "I'm disgusted!" , "It's nasty" , "expression of revulsion" , "How repulsive!" .)