Perfect maybe on horseback? Don't relax (5,2)
I believe the answer is:
tense up
'don't relax' is the definition.
'perfect maybe on horseback?' is the wordplay.
'perfect maybe' becomes 'tense' (perfect tense is an example**).
'on horseback?' becomes 'up'.
'tense'+'up'='TENSE UP'
(Other definitions for tense up that I've seen before include "become uneasy" , "Become stiff through stress or anxiety" , "become edgy" .)