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 "Small fruit for nomads", "sort of fruit", "kind", "Acid purple plum", "jam producer", "Stone fruit for nomads", "Wild plum", "Type of plum", "Small tart plum" and "Plum-coloured".)