Spider monkey's first in van, with hearts trembling (10)
I believe the answer is:
harvestman
'spider' is the definition.
(thesaurus)
'monkey's first in van with hearts trembling' is the wordplay.
'first' indicates taking the first letters.
'in' indicates putting letters inside.
'with' is a charade indicator (letters next to each other).
'trembling' indicates an anagram.
The first letter of 'monkeys' is 'm'.
'van' after 'hearts' is 'heartsvan'.
'heartsvan' with letters rearranged gives 'harvestan'.
'm' placed inside 'harvestan' is 'HARVESTMAN'.
(Other definitions for harvestman that I've seen before include "It's an arachnid" , "one gathers" .)