Bank's first loan is a mixture (5)
I believe the answer is:
blend
'mixture' is the definition.
(I know that blend is a type of mixture)
'bank's first loan' is the wordplay.
'bank's first' becomes 'b' (1st letter of 'bank').
'loan' becomes 'lend' (synonyms).
'b'+'lend'='BLEND'
'is a' acts as a link.
(Other definitions for blend that I've seen before include "Fusion" , "Combine together" , "Mix, stir" , "Mix together" , "Mix and combine" .)