Realise that is five in pain (7)
I believe the answer is:
achieve
'realise' is the definition.
(thesaurus)
'that is five in pain' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'five' becomes 'v' ('V' can be a synonym of 'five').
'in' means one lot of letters goes inside another.
'pain' becomes 'ache' (ache is a kind of pain**).
'ie'+'v'='iev'
'iev' placed within 'ache' is 'ACHIEVE'.
(Other definitions for achieve that I've seen before include "Consummate" , "Reach; be successful" , "Accomplish, gain by effort" , "Manage to get" , "'Accomplish, attain (7)'" .)