Account on a USA service for a tree (6)
I believe the answer is:
acacia
'tree' is the definition.
(acacia is a kind of tree)
'account on a usa service' is the wordplay.
'account' becomes 'ac' (abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'usa service' becomes 'cia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ac'+'a'+'cia'='ACACIA'
'for a' is the link.
Can you help me to learn more?
(Other definitions for acacia that I've seen before include "Wattle genus" , "Plant of the mimosa family" , "Subtropical shrub or tree" , "Tree (wattle or robinia)" , "Tropical shrub" .)