Discover old tabby in heart of glen (6)
I believe the answer is:
locate
'discover' is the definition.
(I've seen this in another clue)
'old tabby in heart of glen' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tabby' becomes 'cat' (I've seen this before).
'in' is an insertion indicator.
'heart of' means to look at the middle letters.
The central letters of 'glen' are 'le'.
'o'+'cat'='ocat'
'ocat' going within 'le' is 'LOCATE'.
(Other definitions for locate that I've seen before include "Set in a particular place" , "Pinpoint in Alecto" , "Fix position of" , "Lay one's hands on (something)" , "Pinpoint, find" .)