Officer behind two others in musical interval (5,5)

'musical interval' is the definition.
(I know that major third is a type of third)

'officer behind two others' is the wordplay.
I cannot quite see how this works, but
'officer' could be 'major' (type of military officer) and 'major' is found within the answer.
The remaining letters 'third' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'in' is the link.

Can you help me to learn more?