Staightforward procedures involving one in poor health (5,7)
I believe the answer is:
Here is my best explanation:
'staightforward' is the definition.
I can't tell whether this definition defines the answer.
'procedures involving one in poor health' is the wordplay.
'procedures' becomes 'plans' (I can't explain this - if you can you should believe this answer much more).
'involving' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'in poor health' becomes 'ailing' (I've seen this before).
'plans' placed around 'i' is 'plains'.
(Other definitions for plain sailing that I've seen before include "smooth progress", "Progress without obstruction", "A breeze", "easy job", "Easy and unobstructed progress", "Trouble-free progress", "effortless progress", "Problem-free progress", "progress made in prairie schooner?" and "No problem (for yachtsman?)".)