Much later astray in empty galaxy (7)
I believe the answer is:
greatly
'much' is the definition.
(I've seen this before)
'later astray in empty galaxy' is the wordplay.
'astray' is an anagram indicator.
'in' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'galaxy' with its centre removed is 'gy'.
'later' with letters rearranged gives 'reatl'.
'reatl' going inside 'gy' is 'GREATLY'.
(Other definitions for greatly that I've seen before include "Very much" , "To a large extent" , "to a considerable extent" .)