Intimate function, close to capacity (4)
I believe the answer is:
cosy
'intimate' is the definition.
(I've seen this before)
'function close to capacity' is the wordplay.
'function' becomes 'cos' (mathematical function).
'close to' suggests the final letters (the 'close' to the word).
The last letter of 'capacity' is 'y'.
'cos'+'y'='COSY'
(Other definitions for cosy that I've seen before include "Warm, secure" , "Comfy and snug" , "With a feeling of comfort and warmth" , "Warm and snug" , "Comfortable; teapot cover" .)