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.
(synonyms)
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" .)