Wallflower becoming less wild, lacking nitrogen (9,6)
I believe the answer is:
shrinking violet
'wallflower' is the definition.
(both can mean a shy person)
'becoming less wild lacking nitrogen' is the wordplay.
'becoming less' becomes 'shrinking'.
'wild' becomes 'violent' (similar in meaning).
'lacking' indicates named letters should be taken away.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'violent' with 'n' taken out is 'violet'.
'shrinking'+'violet'='SHRINKING VIOLET'
(Other definitions for shrinking violet that I've seen before include "wallflower" , "shy type" , "Person not likely to stand out" , "Exaggeratedly shy person" , "Plant not growing" .)