Old man turned and concealed garden pest (5)
I believe the answer is:
aphid
'garden pest' is the definition.
(I've seen this before)
'old man turned and concealed' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'turned' is a reversal indicator.
'and' says to put letters next to each other.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'pa' backwards is 'ap'.
'ap'+'hid'='APHID'
(Other definitions for aphid that I've seen before include "Small plant-sucking insect" , "Greenfly etc" , "Garden pest" , "Blackfly or greenfly" , "little sucker" .)