Determined relative maybe camping outside (9)
I believe the answer is:
insistent
'determined' is the definition.
(insistent can mean determined or tenacious)
'relative maybe camping outside' is the wordplay.
'relative' becomes 'sis' (short for sister).
'maybe camping' becomes 'in tent' (camping is using a tent as shelter).
'outside' means one lot of letters goes inside another.
'sis' put inside 'intent' is 'INSISTENT'.
(Other definitions for insistent that I've seen before include "Constant" , "Unwilling to take "no" for an answer" , "Declaring or demanding emphatically" , "Demanding attention" , "Emphatic" .)