To supply a department requires a lot of shipping (6)
I believe the answer is:
armada
'lot of shipping' is the definition.
'armada' can be an answer for 'lot' (I have seen 'lot of craft' mean 'armada' so perhaps 'lot' could also mean 'armada'). I am not sure about the 'of shipping' bit.
'to supply a department requires a' is the wordplay.
'to supply' becomes 'arm' (to arm someone with something is to supply it).
'department' becomes 'd' (abbreviation eg in MOD Ministry of Defence).
'requires' says to put letters next to each other.
'arm'+'a'+'d'+'a'='ARMADA'
Can you help me to learn more?
(Other definitions for armada that I've seen before include "Fighting unit" , "The Spanish force" , "Large flotilla" , "Navy" , "sea-borne unit" .)