Spotted dog's beginning to follow detective (4)
I believe the answer is:
eyed
'spotted' is the definition.
(I've seen this before)
'dog's beginning to follow detective' is the wordplay.
'beginning to' says to take the initial letters.
'follow' is a charade indicator (letters next to each other).
'detective' becomes 'eye' (I've seen this in other clues).
The initial letter of 'dogs' is 'd'.
'd' put after 'eye' is 'EYED'.
(Other definitions for eyed that I've seen before include "Watched with interest" , "Ogled" , "Stared at" , "Watched, looked at" , "Looked at closely" .)