The bulk is loaded, without mass it cannot take off (7)
I believe the answer is:
ostrich
'mass it cannot take off' is the definition.
I can't judge whether this definition defines the answer.
'the bulk is loaded without' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'bulk' becomes 'os' (I can't justify this - if you can you should believe this answer much more).
'is loaded' becomes 'rich' (similar in meaning).
'without' is an insertion indicator.
'os'+'rich'='osrich'
't' put into 'osrich' is 'OSTRICH'.
Can you help me to learn more?
(Other definitions for ostrich that I've seen before include "World's largest bird" , "Runner" , "Largest living bird" , "Person refusing to face reality" , "Fast winger" .)