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' says the letters should be written in reverse.
'and' says to put letters next to each other.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'pa' written backwards gives '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" .)