Mostly great opening for Olympic movement (5)
I believe the answer is:
largo
'movement' is the definition.
(largo is a kind of movement)
'mostly great opening for olympic' is the wordplay.
'mostly' means to remove the last letter.
'great' becomes 'large' ('large' can be similar in meaning to 'great').
'opening for' indicates taking the first letters.
The first letter of 'olympic' is 'o'.
'large' with its final letter taken away is 'larg'.
'larg'+'o'='LARGO'
(Other definitions for largo that I've seen before include "Handel's piece?" , "Broad and slow (mus.)" , "Slow and dignified, like Handel's composition" , "Key -, Humphrey Bogart film" , "moving slowly" .)