To invest is most successful when holding English capital (5)
I believe the answer is:
beset
'to invest' is the definition.
(thesaurus)
'most successful when holding english capital' is the wordplay.
'most successful' becomes 'best'.
'when holding' is an insertion indicator.
'capital' indicates taking the first letters.
The first letter of 'english' is 'e'.
'best' enclosing 'e' is 'BESET'.
'is' is the link.
(Other definitions for beset that I've seen before include "Attacked from all sides" , "Hem in, afflict" , "Surrounded, under siege" , "Plagued" , "Assail (with problems?)" .)