Construction I had made firm (5)
I believe the answer is:
rigid
'firm' is the definition.
(I've seen this in another clue)
'construction i had' is the wordplay.
'construction' becomes 'rig' (rig is a kind of construction).
'i had' becomes 'I'd' (contraction).
'rig'+'id'='RIGID'
'made' acts as a link.
(Other definitions for rigid that I've seen before include "Incapable of or resistant to bending" , "Uncompromising" , "starched too much?" , "not relaxing" , "unwilling to move" .)