Nervous tension makes rest difficult on board (6)
I believe the answer is:
stress
'nervous tension' is the definition.
(I've seen this before)
'rest difficult on board' is the wordplay.
'difficult' is an anagram indicator.
'on board' becomes 'ss' (I've seen this in other clues).
'rest' is an anagram of 'stre'.
'stre'+'ss'='STRESS'
'makes' acts as a link.
(Other definitions for stress that I've seen before include "result of being overworked" , "Lay emphasis on - it's a strain" , "nervous tension" , "Physical or mental over-exertion" , "State mental or emotional strain" .)