Trembling aspens holding unknown mental connection (7)
I believe the answer is:
synapse
'mental connection' is the definition.
(synapses relay nerve impulses)
'trembling aspens holding unknown' is the wordplay.
'trembling' indicates anagramming the letters.
'holding' means one lot of letters goes inside another.
'unknown' becomes 'y' (algebra).
'aspens' anagrammed gives 'snapse'.
'snapse' enclosing 'y' is 'SYNAPSE'.
(Other definitions for synapse that I've seen before include "Brain connection" , "Nerve junction" , "Where two nerve cells meet" , "Nerve cell connector" , "It links cells" .)