Pip lassoes head of troublesome horse (5)
I believe the answer is:
steed
'horse' is the definition.
(I know that steed is a type of warhorse)
'pip lassoes head of troublesome' is the wordplay.
'pip' becomes 'seed' (I've seen this before).
'lassoes' is an insertion indicator (inserted letters are lassoed or caught).
'head of' indicates taking the first letters.
The first letter of 'troublesome' is 't'.
'seed' going around 't' is 'STEED'.
(Other definitions for steed that I've seen before include "Warhorse" , "Powerful horse" , "Trusty horse" , "Stallion" , "Spirited horse" .)