Tree line on middle of bank (5)
I believe the answer is:
rowan
'tree' is the definition.
(rowan is a kind of tree)
'line on middle of bank' is the wordplay.
'line' becomes 'row' (row is a kind of line**).
'on' is a charade indicator (letters next to each other).
'middle of' means to look at the middle letters.
The centre of 'bank' is 'an'.
'row'+'an'='ROWAN'
(Other definitions for rowan that I've seen before include "Sort of ash" , "Red-berried tree" , "Mountain ash (at Lambeth!)" , "Tree with orange-red berry-like fruits" , "Man" .)