Unknown Berlin figure: I'm not sure he has a craft (7)
I believe the answer is:
yachter
'he has a craft' is the definition.
'unknown berlin figure i'm not sure' is the wordplay.
'unknown' becomes 'y' (algebra).
'berlin figure' becomes 'acht' ('eight' in German).
'i'm not sure' becomes 'er' (expression of hesitation).
'y'+'acht'+'er'='YACHTER'
(Other definitions for yachter that I've seen before include "person on board?" , "sailor" .)