Plant account penned by American spies (6)
I believe the answer is:
acacia
'plant' is the definition.
The answer is an example.
'account penned by american spies' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'penned by' means one lot of letters goes inside another.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'spies' becomes 'cia' (Central Intelligence Agency).
'a'+'cia'='acia'
'ac' inserted inside 'acia' is 'ACACIA'.
(Other definitions for acacia that I've seen before include "ornamental tree" , "Flowering shrub of the pea family" , "Spiny tree of warm climates" , "flowering plant" , "Plant such as the Australian wattle" .)