World leader in marketing ballooning (8)
I believe the answer is:
swelling
'ballooning' is the definition.
(thesaurus)
'world leader in marketing' is the wordplay.
'leader' indicates taking the first letters.
'in' indicates putting letters inside.
'marketing' becomes 'selling' (synonyms).
The initial letter of 'world' is 'w'.
'w' put inside 'selling' is 'SWELLING'.
(Other definitions for swelling that I've seen before include "Nodule" , "getting bigger" , "Bulge" , "Tumour" , "Growth" .)