Eat little mushroom that’s turned darkish at the centre (4)
I believe the answer is:
peck
'eat' is the definition.
(pecking is a kind of eating)
'little mushroom that's turned darkish at the centre' is the wordplay.
'little mushroom' becomes 'cep' (type of mushroom).
'that's turned' is a reversal indicator.
'at the centre' means to look at the middle letters.
The central letter of 'darkish' is 'k'.
'cep' in reverse letter order is 'pec'.
'pec'+'k'='PECK'
(Other definitions for peck that I've seen before include "Kiss; part of bushel" , "Bite lightly; a dry measure" , "Attack with beak" , "Strike with beak" , "Large number" .)