Groom not fully prepared, suffering head loss under pressure (5)
I believe the answer is:
preen
'groom' is the definition.
(a bird grooms itself by preening feathers)
'fully prepared suffering head loss under pressure' is the wordplay.
'fully prepared suffering head loss' becomes 'reen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'pressure' becomes 'p' (abbreviation).
'reen' put after 'p' is 'PREEN'.
'not' acts as a link.
Can you help me to learn more?
(Other definitions for preen that I've seen before include "Dress or smooth, feathers say" , "demonstrate pride" , "Groom with elaborate care" , "to give himself a pat on the back" , "setdown straight?" .)