Oddly calmer after boats at sea in difficulty (8)
I believe the answer is:
obstacle
'difficulty' is the definition.
(I know that obstacle is a type of balk)
'oddly calmer after boats at sea' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'after' says to put letters next to each other.
'at sea' indicates anagramming the letters.
The alternating letters of 'calmer' are 'cle'.
'boats' is an anagram of 'obsta'.
'cle' put after 'obsta' is 'OBSTACLE'.
'in' acts as a link.
(Other definitions for obstacle that I've seen before include "Barrier or deterrent" , "bar" , "Hurdle, impediment" , "Thing that obstructs progress" , "Blockage, hindrance" .)