Three times as many appear in a quiver (7)
I believe the answer is:
tremble
'a quiver' is the definition.
(thesaurus)
'three times as many appear in' is the wordplay.
'three times as many' becomes 'treble' (I've seen this before).
'appear' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'treble' enclosing 'm' is 'TREMBLE'.
Can you help me to learn more?
(Other definitions for tremble that I've seen before include "sign of nerves" , "Look nervous" , "Quake in one's shoes" , "Shiver, quake" , "Shake involuntarily" .)