One trembles, sent without cover after killer (5)
I believe the answer is:
aspen
'one trembles' is the definition.
(as in a trembling aspen tree)
'sent without cover after killer' is the wordplay.
'without cover' says to take the centre (the letters which cover up the word are removed).
'after' means one lot of letters go next to another.
'killer' becomes 'asp' (Kind of poisonous snake).
The middle letters of 'sent' are 'en'.
'en' after 'asp' is 'ASPEN'.
(Other definitions for aspen that I've seen before include "Colorado winter sports town" , "Colorado ski resort - tree" , "Tree that quivers" , "Trembling poplar tree" , "Type of poplar" .)