Foreign seaman — a terrible rascal (6)
I believe the answer is:
lascar
'foreign seaman' is the definition.
'lascar' can be an answer for 'seaman' (I've seen this before). I'm not sure about the 'foreign' bit.
'a terrible rascal' is the wordplay.
'a terrible' is an anagram indicator.
'rascal' with letters rearranged gives 'LASCAR'.
Can you help me to learn more?
(Other definitions for lascar that I've seen before include "Indiaman Jack" , "Camp-follower" , "Eastern sailor" , "Rascal (anag) - oriental seaman" , "Asian sailor" .)