Gloomy and could become more so (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this in another clue)
'could become more so' is the wordplay.
'could become' indicates an anagram.
'more'+'so'='moreso'
'moreso' with letters rearranged gives 'MOROSE'.
'and' is the link.
(Other definitions for morose that I've seen before include "Sad" , "Miserable" , "Surly and unforthcoming" , "blue" , "Peevish and gloomy" .)