Marine guards Queen, returning debt that will grow massively (7)
I believe the answer is:
sequoia
'that will grow massively' is the definition.
(sequoias are tall trees)
'marine guards queen returning debt' is the wordplay.
'marine' becomes 'sea' (both can mean 'of the sea').
'guards' is an insertion indicator.
'queen' becomes 'q' (abbreviation used in chess).
'returning' says the letters should be written backwards.
'debt' becomes 'iou'.
'iou' written backwards gives 'uoi'.
'q'+'uoi'='quoi'
'sea' enclosing 'quoi' is 'SEQUOIA'.
(Other definitions for sequoia that I've seen before include "Californian conifer" , "big tree" , "Giant Calif. tree" , "California redwood tree" , "Giant conifer" .)