Need to trim the rowan tree (4)
I believe the answer is:
want
'need to' is the definition.
(thesaurus)
'trim the rowan tree' is the wordplay.
'trim' says the answer is hidden in the clue (I've seen 'trimming' mean this).
'WANT' can be found hidden inside 'rowan tree'.
(Other definitions for want that I've seen before include "Need or desire" , "Crave" , "poverty" , "Desire - deficiency" , "Wish" .)