Be slow to grasp new question without agitation (8)
I believe the answer is:
Here is my best explanation:
'without agitation' is the definition.
'be slow to grasp new question' is the wordplay.
'be slow' becomes 'trail'.
'to grasp' is an insertion indicator.
'new' becomes 'n'.
'question' becomes 'qu'.
'trail' placed around 'nqu' is 'TRANQUIL'.
(Other definitions for tranquil that I've seen before include "Calm and serene", "Still", "composed", "not making waves", "Serenely quiet or peaceful", "Serene", "Calm, placid, still", "Calm and undisturbed, at peace", "Peaceful and quiet, not agitated" and "Undisturbed".)