Partner of gangster from America caught a slug? (7)
I believe the answer is:
mollusc
'a slug?' is the definition.
(slug is a kind of mollusc)
'partner of gangster from america caught' is the wordplay.
'partner of gangster' becomes 'moll' (I can't explain this - if you can you should give a lot more credence to this answer).
'from america' becomes 'US' (United States).
'caught' becomes 'c' (cricket abbreviation).
'moll'+'us'+'c'='MOLLUSC'
Can you help me to learn more?
(Other definitions for mollusc that I've seen before include "Invertebrate such as a snail" , "sluggish person" , "Gastropod or cephalopod" , "Invertebrate with a soft unsegmented body, usually inside a shell" , "Shelled invertebrate" .)