One of two giants crushing head of reckless spirit? (4)
I believe the answer is:
grog
'spirit?' is the definition.
(I've seen this before)
'one of two giants crushing head of reckless' is the wordplay.
'one of two giants' becomes 'gog' (Gog and Magog are mythical giants).
'crushing' means one lot of letters goes inside another (inserted letters are crushed by others).
'head of' suggests taking the first letters.
The initial letter of 'reckless' is 'r'.
'gog' going around 'r' is 'GROG'.
(Other definitions for grog that I've seen before include "Traditional naval drink" , "Drink of spirits mixed with water" , "Old sailors' tipple" , "Drink of spirits and water liked by sailors" , "Old naval drink" .)