Trio in plane possibly circling capital of Haiti (5)
I believe the answer is:
three
'trio' is the definition.
(synonyms)
'plane possibly circling capital of haiti' is the wordplay.
'plane possibly' becomes 'tree' (plane is a type of tree).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'capital of' says to take the initial letters.
The first letter of 'haiti' is 'h'.
'tree' going around 'h' is 'THREE'.
'in' acts as a link.
(Other definitions for three that I've seen before include "So many 12s today" , "Bears or blind mice" , "Number of strikes before you're out" , "Number of Dumas's musketeers" , "Number of times N appears in this completed grid" .)