Ribbon stuck initially on tree (4)
I believe the answer is:
sash
'ribbon' is the definition.
(I've seen this before)
'stuck initially on tree' is the wordplay.
'initially' suggests taking the first letters.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'tree' becomes 'ash' (ash is a kind of tree).
The initial letter of 'stuck' is 's'.
's'+'ash'='SASH'
(Other definitions for sash that I've seen before include "Baldrick" , "type of cord?" , "Type of sliding window" , "Part of a window - decoration" , "Framework holding window panes" .)