Made stable lad heading away head in immediately to stop horse (8)
I believe the answer is:
steadied
'made stable' is the definition.
(I've seen this before)
'lad heading away head in immediately to stop horse' is the wordplay.
'heading away' means to remove the first letter (the word's 'head' is removed).
'head in' indicates taking the first letters.
'to stop' is an insertion indicator (inserted letters plug or stop a gap).
'horse' becomes 'steed' (steed is a kind of horse).
The first letter of 'immediately' is 'i'.
'lad' with its initial letter taken off is 'ad'.
'ad'+'i'='adi'
'adi' placed inside 'steed' is 'STEADIED'.
(Other definitions for steadied that I've seen before include "Made firm and balanced" , "calmed down" , "made stable" , "Supported, controlled" , "Braced" .)