Prepare to be in Paris during grand day out (3,5)
I believe the answer is:
get ready
'prepare' is the definition.
(synonyms)
'be in paris during grand day out' is the wordplay.
'be in paris' becomes 'etre' (I've seen this in another clue).
'during' is an insertion indicator.
'grand' becomes 'g' (G is short for grand as in a thousand).
'out' indicates an anagram (out can mean wrong or inaccurate).
'g'+'day'='gday'
'gday' is an anagram of 'gady'.
'etre' going inside 'gady' is 'GET READY'.
'to' is the link.
(Other definitions for get ready that I've seen before include "third of three" , "Prepare oneself" .)