Use lead on excitable dinosaur going the wrong way (5)
I believe the answer is:
exert
'use' is the definition.
(exerting is a kind of using)
'lead on excitable dinosaur going the wrong way' is the wordplay.
'lead on' indicates taking the first letters.
'dinosaur' becomes 'T Rex' (abbreviation for Tyrannosaurus Rex).
'going the wrong way' says the letters should be written in reverse.
The initial letter of 'excitable' is 'e'.
'trex' backwards is 'xert'.
'e'+'xert'='EXERT'
(Other definitions for exert that I've seen before include "Apply (pressure)" , "Bring into use, strength or influence" , "Make a great effort" , "Cause (oneself) to make a strenuous effort" , "Bring (force) to bear" .)