Old bachelor's remedy, not easily understood (7)
I believe the answer is:
obscure
'not easily understood' is the definition.
(I've seen this before)
'old bachelor's remedy' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bachelor's' becomes 'B's' (abbreviation e.g. BA=Bachelor of Arts).
'remedy' becomes 'cure' (synonyms).
'o'+'bs'+'cure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Insignificant" , "Enigmatic (like Jude?)" , "Recondite, abstruse" , "Dull or impenetrable" , "Dim, indistinct or not well known" .)