Pull together on an ash tree (5)
I believe the answer is:
rowan
'ash tree' is the definition.
'rowan' can be an answer for 'tree' (type of tree). I am unsure of the 'ash' bit.
'pull together on an' is the wordplay.
'pull together' becomes 'row' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'row'+'an'='ROWAN'
Can you help me to learn more?
(Other definitions for rowan that I've seen before include "Canterbury" , "Man" , "-- Williams, theologian" , "The mountain ash tree" , "Red-berried tree" .)