Tension disturbed rest on board (6)
I believe the answer is:
stress
'tension' is the definition.
(I know that tension can be written as stress)
'disturbed rest on board' is the wordplay.
'disturbed' is an anagram indicator (the order of letters is disturbed).
'on board' becomes 'ss' (I've seen this in other clues).
'rest' anagrammed gives 'stre'.
'stre'+'ss'='STRESS'
(Other definitions for stress that I've seen before include "Cause of crack" , "Tension, pressure" , "State mental or emotional strain" , "Physical or mental over-exertion" , "'Worry, pressure (6)'" .)