Copper and aluminium encasing part of the forearm (7)

'of the forearm' is the definition.

'copper and aluminium encasing part' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'and' means one lot of letters go next to another.
'aluminium' becomes 'al' ('Al' can be a synonym of 'aluminium').
'encasing' indicates putting letters inside.
'part' becomes 'bit' (bit is a kind of part**).
'cu'+'al'='cual'
'cual' placed around 'bit' is 'CUBITAL'.