Sad, and apt to become more so (6)
I believe the answer is:
morose
'sad' is the definition.
(I've seen this before)
'become more so' is the wordplay.
'become' indicates anagramming the letters.
'more'+'so'='moreso'
'moreso' is an anagram of 'MOROSE'.
'and apt to' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for morose that I've seen before include "Sullen and moody" , "Sad" , "Sullen and brooding" , "Sullen, crabby" , "Down in the dumps" .)