Gets bigger flags, dropping one (5)
I believe the answer is:
rises
'gets bigger' is the definition.
(rise can mean to grow or increase)
'flags dropping one' is the wordplay.
'flags' becomes 'raises' (to flag something is to raise it as an issue).
'dropping' indicates named letters should be taken away.
'one' becomes 'a' (a thing is one thing).
'raises' with 'a' removed is 'RISES'.
(Other definitions for rises that I've seen before include "Comes or goes upward" , "Gets up" , "Moves upwards" , "Ascends" , "Moves upward" .)