Surprise for eighth anniversary: pastry with side of lettuce (7)
I believe the answer is:
startle
'surprise' is the definition.
(to startle someone is to surprise or shock them)
'eighth anniversary pastry with side of lettuce' is the wordplay.
'eighth anniversary' becomes 's' (8th letter of 'anniversary').
'pastry' becomes 'tart' (type of pastry).
'with' means one lot of letters go next to another.
'side of' suggests removing the centre (I've seen 'sides of' mean this (only the letters on the sides remain)).
'lettuce' with its middle removed is 'le'.
's'+'tart'+'le'='STARTLE'
'for' acts as a link.
(Other definitions for startle that I've seen before include "Alarm suddenly" , "Surprise with fright" , "Cause to jump" , "Take by surprise" , "Frighten suddenly" .)