Certainly US force catastrophic under head of operations (2,6)
I believe the answer is:
of course
'certainly' is the definition.
(both can be used to answer in the affirmative)
'us force catastrophic under head of operations' is the wordplay.
'catastrophic' indicates anagramming the letters.
'under' says to put letters next to each other.
'head of' indicates taking the first letters.
The first letter of 'operations' is 'o'.
'us'+'force'='usforce'
'usforce' anagrammed gives 'fcourse'.
'fcourse' after 'o' is 'OF COURSE'.
(Other definitions for of course that I've seen before include "Definitely" , "Naturally, as one would expect" , "About a series of lessons" , "without question" , "need I say" .)