Fish popular at present with millions (6)
I believe the answer is:
minnow
'fish' is the definition.
(minnow is a kind of fish)
'popular at present with millions' is the wordplay.
'popular' becomes 'in'.
'at present' becomes 'now' ('now' can be a synonym of 'at present').
'with' is a charade indicator (letters next to each other).
'millions' becomes 'm' (abbreviation).
'in'+'now'='innow'
'innow' put after 'm' is 'MINNOW'.
(Other definitions for minnow that I've seen before include "Not much of a catch?" , "Tiny freshwater fish" , "Small person" , "A fish; insignificant person" , "Very little fish" .)