Smallest, hardest

across

1 US tax pro (abbr) (3)

4 Amusing person (colloq) (4)

5 A framework (4)

6 Timber tree (3)

down

1 A peasant (obs) (4)

2 Stiffly formal (4)

3 Tack on (3)

4 Horse-drawn vehicle

(abbr) (3)