They are, in the main, army or company retreats with first-class appliances (9)
I believe the answer is:
octopuses
'they are in the main army' is the definition.
The definition suggests an adverb but the answer is not.
'company retreats with first-class appliances' is the wordplay.
'company' becomes 'co' (abbreviation).
'retreats' is a reversal indicator (I've seen 'retreating' mean this).
'with' says to put letters next to each other.
'first-class' becomes 'top'.
'appliances' becomes 'uses' (I can't justify this - if you can you should give a lot more credence to this answer).
'co' backwards is 'oc'.
'oc'+'top'+'uses'='OCTOPUSES'
'or' is the link.
Can you help me to learn more?
(Other definitions for octopuses that I've seen before include "With eight long tentacles, they live in the deep" , "Armed sea creatures" , "Sea molluscs with eight arms" , "influential organisations" , "Multi-armed sea creatures" .)