Fret about a conifer (6)
I believe the answer is:
repine
'fret' is the definition.
(I've seen this before)
'about a conifer' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a conifer' becomes 'pine' (pine is a kind of conifer).
're'+'pine'='REPINE'
(Other definitions for repine that I've seen before include "Fret, be discontented" , "Express discontent, fret" , "Fret, about a tree perhaps?" , "Be in a state of anxiety" , "Plain" .)