East Ender seen to crack under strain (5)
I believe the answer is:
tense
'under strain' is the definition.
(I've seen this before)
'east ender seen to crack' is the wordplay.
'ender' says to take the final letters (I've seen 'end' mean this).
'to crack' indicates anagramming the letters (I've seen 'cracked' mean this).
The last letter of 'east' is 't'.
'seen' is an anagram of 'ense'.
't'+'ense'='TENSE'
(Other definitions for tense that I've seen before include "Form of verb showing time" , "Attribute of verbs signalling time" , "under pressure" , "Strained - form of verb" , "Nervous or tight" .)