Fungal infection's beginning to spread, becoming lucrative (7)
I believe the answer is:
gainful
'lucrative' is the definition.
(thesaurus)
'fungal infection's beginning to spread' is the wordplay.
'beginning to' suggests taking the first letters.
'spread' is an anagram indicator.
The first letter of 'infections' is 'i'.
'fungal'+'i'='fungali'
'fungali' with letters rearranged gives 'GAINFUL'.
'becoming' is the link.
(Other definitions for gainful that I've seen before include "Bringing in money" , "Profitable and productive" .)