Spies appear after hiding odd bits of larch and oak tree (6)
I believe the answer is:
acacia
'tree' is the definition.
(acacia is a kind of tree)
'spies appear after hiding odd bits of larch and oak' is the wordplay.
'spies' becomes 'CIA' (Central Intelligence Agency).
'appear after' means one lot of letters go next to another.
'hiding odd bits of' indicates alternate letters (letters in odd-numbered positions removed).
'and' is a charade indicator (letters next to each other).
'oak' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
The alternating letters of 'larch' are 'ac'.
'ac'+'a'='aca'
'cia' after 'aca' is 'ACACIA'.
(Other definitions for acacia that I've seen before include "Flowering tree or shrub" , "flowering plant" , "Type of shrub or tree" , "Spiny tree or shrub" , "ornamental tree" .)