Evasive about son being comfortable (4)
I believe the answer is:
Here is my best explanation:
'comfortable' is the definition.
('cosy' can be similar in meaning to 'comfortable')
'evasive about son' is the subsidiary indication.
'evasive' becomes 'coy'.
'about' means one lot of letters goes inside another.
'son' becomes 's'.
'coy' enclosing 's' is 'cosy'.
'being' is the link.
(Other definitions for cosy that I've seen before include "Comfortable; teapot cover", "Warm, secure", "Warm and snug", "Comfortable, warm", "Teapot cover", "Snug", "Warm and comfortable", "With a feeling of comfort and warmth", "Cover for a teapot" and "Comfy and snug".)