Fall short of goal getting junior to a church one evening (12)
I believe the answer is:
'fall short of goal' is the definition.
(fail to achieve a goal)
'junior to a church one evening' is the wordplay.
'junior' becomes 'under' (I've seen this in other clues).
'to' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
'one' becomes 'I' (Roman numeral).
'evening' becomes 'eve' (synonyms).
'getting' is the link.
(Another definition for underachieve that I've seen is " Do less well than expected".)