British sailors gathering round African tree (6)
I believe the answer is:
baobab
'african tree' is the definition.
(I've seen this before)
'british sailors gathering round' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sailors' becomes 'ab ab' (two instances of the abbreviation for able seaman).
'gathering' means one lot of letters goes inside another.
'round' becomes 'o' (round can mean a round object).
'abab' enclosing 'o' is 'aobab'.
'b'+'aobab'='BAOBAB'
(Other definitions for baobab that I've seen before include "Tree indigenous to Africa and Australia" , "Exotic tree" , "Monkey-bread tree" , "Gigantic tropical tree with very thick trunk" , "African fruit tree" .)