Realise that is very concealed by pain (7)
I believe the answer is:
achieve
'realise' is the definition.
(thesaurus)
'that is very concealed by pain' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'very' becomes 'v' (abbreviation).
'concealed by' indicates putting letters inside.
'pain' becomes 'ache' (ache is a kind of pain).
'ie'+'v'='iev'
'iev' going within 'ache' is 'ACHIEVE'.
(Other definitions for achieve that I've seen before include "Attain successfully" , "Attain (goal)" , "win" , "Successfully conclude" , "Finish" .)