... on tenterhooks since meeting head of Givenchy (4)
I believe the answer is:
agog
'on tenterhooks' is the definition.
(I've seen this before)
'since meeting head of givenchy' is the wordplay.
'since' becomes 'ago' (I've seen this before).
'meeting' means one lot of letters go next to another.
'head of' says to take the initial letters.
The first letter of 'givenchy' is 'g'.
'ago'+'g'='AGOG'
(Other definitions for agog that I've seen before include "Wide-eyed in anticipation" , "with eyes on stalks" , "Very eager (to see/hear)" , "On tenterhooks" , "Very eager (in anticipation)" .)