Barriers no problem for plum (6)
I believe the answer is:
Here is my best explanation:
'plum' is the definition.
(damson is a kind of plum)
'barriers no problem' is the subsidiary indication.
'barriers' becomes 'dams' (dam is a kind of barrier).
'problem' indicates anagramming the letters.
'no' is an anagram of 'on'.
'for' is the link.
(Other definitions for damson that I've seen before include "Sort of plum", "Wild plum", "Fruit for nomads", "Small fruit for nomads", "Small dark-purple plum", "Nomads (anag)", "Kind of plum for nomads", "Small plum-like fruit", "Plum-like fruit" and "Ma, Don's a fruit".)