At the Boar's Head, calls for revels (5)

'revels' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot understand how they can define each other.

'at the boar's head calls for' is the wordplay.
'at' means one lot of letters go next to another.
'head' says to take the initial letters.
'calls for' becomes 'asks' ('ask' can be a synonym of 'call for').
The initial letter of 'boars' is 'b'.
'b' next to 'asks' is 'BASKS'.

Can you help me to learn more?