Emphatic type with male mice hearts (6)
I believe the answer is:
italic
'emphatic type' is the definition.
(typeface used for emphasis)
'with male mice hearts' is the wordplay.
'with' indicates putting letters inside.
'male' becomes 'tali' (I can't justify this - if you can you should give a lot more credence to this answer).
'hearts' means to look at the middle letters (I've seen 'heart' mean this).
The middle letters of 'mice' are 'ic'.
'tali' placed into 'ic' is 'ITALIC'.
(Other definitions for italic that I've seen before include "Relating to slanting handwriting or type" , "(Of text) sloping" , "Print style used for emphasis, titles of books, etc" , "from Rome?" , "Printing style used for emphasis" .)