Letter spelled out work in detail (6)
I believe the answer is:
'in detail' is the definition.
(eg reading deeply on a topic)
'letter spelled out work' is the wordplay.
'letter spelled out' becomes 'dee' (the spelled-out name for the letter D).
'work' becomes 'ply' (I've seen this before).
(Other definitions for deeply that I've seen before include "With intense feeling", "Profoundly", "fathoms down?", "Thoroughly", "In a profound way".)