Tremble when one's supporting one queen against another (6)
I believe the answer is:
quiver
'tremble' is the definition.
(quivering is a kind of trembling)
'queen against another' is the wordplay.
'queen' becomes 'qu'.
'against' means one lot of letters go next to another.
'another' becomes 'iver' (I am not sure about this - if you are sure you should believe this answer much more).
'qu'+'iver'='QUIVER'
'when one's supporting one' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for quiver that I've seen before include "Arrow case" , "Tremble and shake, or receptacle for arrows" , "Arrow-holder; shake" , "Receptacle in which archer keeps arrows" , "Tremble - case for arrows" .)