Monster that could make you weak at first and possibly nervy (6)
I believe the answer is:
wyvern
'monster' is the definition.
(I know that wyvern is a type of dragon)
'weak at first and possibly nervy' is the wordplay.
'at first' says to take the initial letters.
'and' says to put letters next to each other.
'possibly' indicates anagramming the letters.
The initial letter of 'weak' is 'w'.
'nervy' with letters rearranged gives 'yvern'.
'w'+'yvern'='WYVERN'
'that could make you' is the link.
(Other definitions for wyvern that I've seen before include "Heraldic device" , "Heraldic dragon with two legs and two wings" , "Winged two-legged dragon (heraldry)" , "1950s Vauxhall model" , "Dragon with wings in medieval heraldry" .)