Fool at the first opportunity losing head (3)
I believe the answer is:
sap
'fool' is the definition.
(I know that fool can be written as sap)
'at the first opportunity losing head' is the wordplay.
'at the first opportunity' becomes 'ASAP' (as soon as possible).
'losing head' means to remove the first letter.
'asap' with its first letter taken away is 'SAP'.
(Other definitions for sap that I've seen before include "Enervate" , "Fluid in plant stems" , "Gradually weaken (a person's strength)" , "Moisture in tree or informal dunce" , "Imbecile" .)