A chief accountant joining US agency: this grows thorny (6)
I believe the answer is:
acacia
'this grows thorny' is the definition.
The definition suggests a singular noun which matches the answer.
'a chief accountant joining us agency' is the wordplay.
'chief accountant' becomes 'ca' (this could be a standard abbreviation of which I'm unaware).
'joining' is a charade indicator (letters next to each other).
'us agency' becomes 'cia' (I've seen this before).
'a'+'ca'+'cia'='ACACIA'
Can you help me to learn more?
(Other definitions for acacia that I've seen before include "Spiny tree of warm climates" , "Garden shrub, also known as wattle" , "Plant like wattle" , "source of gum arabic" , "Australian shrub" .)