Staunch (4)
(Other definitions for avid that I've seen before include "Greedy or eagerly desirous" , "Zealous, enthusiastic" , "Fervent" , "hungry" , "deeply enthusiastic" .)
'staunch' is the definition.
(I know that staunch can be written as stem)
This is the entire clue.
(Other definitions for stem that I've seen before include "Main stalk of a plant" , "Block" , "Flower's stick" , "Hold back - stock - check" , "Slender upright part, of flower or wineglass" .)