Fastener securing tree unfortunately damaged, initially (4)
I believe the answer is:
stud
'fastener' is the definition.
(I've seen this before)
'securing tree unfortunately damaged initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'securing tree unfortunately damaged' is 'STUD'.
(Other definitions for stud that I've seen before include "Specified arrangement" , "Horse-breeding establishment; jewellery item" , "Place of breeding" , "Horse-breeding farm" , "Farm for breeding horses" .)