Pressure forced sister to carry on (7)
I believe the answer is:
persist
'carry on' is the definition.
(I've seen this before)
'pressure forced sister' is the wordplay.
'pressure' becomes 'p' (abbreviation**).
'forced' is an anagram indicator.
'sister' is an anagram of 'ersist'.
'p'+'ersist'='PERSIST'
'to' is the link.
(Other definitions for persist that I've seen before include "Continue firmly or obstinately" , "Keep trying" , "Show doggedness" , "Prevail" , "Refuse to stop" .)