Rocky mostly very upset Ali is possibly The Greatest (11)
I believe the answer is:
superlative
'greatest' is the definition.
(synonyms)
'rocky mostly very upset ali' is the wordplay.
'rocky' indicates an anagram (letters rocked or thrown around).
'mostly' means to remove the last letter (most but not all of the word).
'very' with its final letter taken off is 'ver'.
'ver'+'upset'+'ali'='verupsetali'
'verupsetali' with letters rearranged gives 'SUPERLATIVE'.
'is possibly the' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for superlative that I've seen before include "Unsurpassed" , "Word often ending in '-est'" , "Of the highest degree or quality" .)